Welcome, Meyer.
I suggest you go to any woods close to where your trees are, collect some mushrooms (ground-growing ones, and preferably not poisonous), blend them up with non-chlorinated water, and pour the slurry around your trouble trees. Alternately, I would take whatever mushrooms you eat and trim the butts, reserving them in the refrigerator until you have enough to make another slurry. Or you could just buy mushrooms from the store and make a mushroom slurry from them.
Trees prefer a soil that leans further toward a fungal-dominated biology, and if you're growing mostly grass and vegetables, which are more bacterially than fungally active, the trees might be lacking the support networks provided by the fungi.
More information than you could possibly ever use is available in
Dr. Redhawk's Wiki of Epic Soil Threads. I suggest you skim it, and read in detail whatever might apply to your situation.
But let us know how it goes, and good luck.
-CK
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein