When a tree weakens, what to do?
Lack of nutrients, or illnesses. Lack of nutrients requires soil analysis, and usually the deficit is found withing the color of the leaves, although many deficits look alike. The major illness that causes tree to weakens is instead wood rot...At least in dry areas, whereas in colder climates bacteria can be worse, however bacteria and viruses don't have solutions nowadays...
Plants are full of antioxidants, and other compounds that deter different kind of illnesses. Be it either for lack of nutrients or illnesses, applying fertilizer is always the first course of action. What fertilizer? As I said before it kind of needs proper soil analysis, however most fertilizers sold, provide a good mix of the most important elements. Composite or simple? Both is the correct answer. Instead of providing 100% in-organic, consider a 1/2 split or a 2/3 split in favor of organic fertilizers.
"I tried putting fertilizers but it hasn't had any effect"
When you apply fertilizer on large terrains, you are betting that the fertilizer reaches the plant. There are a few problems with this bet:
When plants are small, roots are also small, unless the cultivation is super-intensive the chance of fertilizers not reaching the plants is quite high.
Percolation is a thing. In-organic fertilizer dissolves in water, when it rains, the fertilizer goes with water. You rely on plants to hold as much water as they can with their hair roots system, but whatever is between plants is usually lost. This is where in-organic fertilizer can help, since they can act as a sponge helping with inorganic elements retention.
When the mountain doesn't go...Weakened trees might have trouble absorbing nutrients, if you want to be sure that the plant has got the best chance at nutrient absorption, you need to uncover the superficial part of the root system, and put fertilizer side by side the roots, making sure to not exaggerate. Once fertilizer is posed, cover again and give it some small doses of water to relieve the stress from the partial uproot event. Remember that not all roots are equal, some are active and absorb nutrients, other only serve as water pipelines, it's important to choose the right one, or do a 360* coverage, although you would be paying the higher cost.
If the plant manages to absorb the nutrients, it will have a chance at deterring or isolating pathogens and resume its normal growth.
Large plants, have had a long life and might have survived different waves of pathogens each wave weakening the plant a little bit. Arm yourself with a drill, a mix of in-organic fertilizer, mycorrhizae and trichoderma, and a syringe. Lets assume that you tried fertilizers, but the plant doesn't respond, maybe it has sprouted new branches from its base, but its top is still slim and rachitic. You have to find out were small (or large) pockets of wood rot reside. Usually if the rot is large, you would remove all of it with a chainsaw, however this interventions are quite stressful for the plant, and almost never remove the rotten wood completely. Your goal with infiltration is to build a trait of canals along the full stump of the tree up until half its trunk. This way you have a complete coverage of potential wood rotting pockets. How large should the canals be? It depends on the size of the tree, start with half a centimeter. In order to achieve 100% coverage, canals must be drilled all around the tree. Their distance and their numbers also depends on the tree size. Always drill along the concavities of the bark, to avoid damaging the younger and active xylematic conduits. 
Once canals are drilled, its syringe time. Preparing the mixtures:
One of your goals is to stimulate growth, therefore you want to use root hormones and fertilizers that promote vigour, like urea. Mix these with water and some soil, and apply them inside the canals with the syringe. (The syringe should be quite large, to allow for 50-100cm depths). Don't apply it to all the canals, only to the ones closer to the ground, around the stump, that you think are the oldest.
The second mix is the one with mycorrhizae, mix them with some water and soil, and push it along the remaining
canals along the stump base.
The last mix with trichoderma should cover all the upwards canals.
Repeat this process at least twice (once in spring, once in autumn). The second time, however, only use myco and trico, splitting the canals 50/50 between them. Don't put myco and trico in the same mix. The goal of infiltration is to restructure the plant xylem to work around pathogens by building an armor with synergistic fungi. There should be some results within 2 years.
Roots and foliage are always in balance. When one grows, the other follows.
You can try to change the genetic material of the foliage with something more vigorous. You are betting that the higher metabolism of a more vigorous cultivar can induce renewed root growth.
Another approach is too apply a rootstock to...the roots? Roots between different trees can merge together to form synergistic sub-terrean networks, but you don't need to reach for other trees. You can try to apply rootstocks from more vigorous cultivar directly to the stump (by digging a little like you did for applying fertilizer), aid yourself with root hormones used to stimulate rooting from plants cuttings.
Results depend on how vigorous the cultivar (and the applied rootstocks) are, in general at least 3 years.
This is the most invasive approach, you have to perform a deep plow-through all around the tree (at least 1 meter deep) as close to the stump as possible. Your goal is to cut all the deepest roots that you can, such that the tree is forced to sprout new ones if it wants to stay alive, boost its chance again with a mix of mycorrhizae and trichoderma, this time you would be applying them on a wider scale, with shoulder carried pump and spraying all around the dug terrain over the uncovered stump. After spraying cover with soil again, ideally mixing it with some breach, in order to keep the soil porous and preventing it from compacting too much, such that new roots will have an easier time growing. 
The plant should respond to this last attempt in a few months, either positively or negatively. If the outcome negative, sorry, its time to consider planting something else.
