Sun is a plant's sole energy source. Those fuel all other life processes in the plant. Check your garden for hours of sun during the growing season, then check your plants in a garden encyclopedia to see if they are getting the light they need. Full sun plants need six or more hours of shadowless sunlight every day. Half sun and half shade species must have four to six hours of sun.
Shade plants require at least two hours of sun each day.
Water is probably another key to this puzzle. We had copious rain last year, so much more than usual that many plants competing with trees for their water grew the best their gardeners had ever seen. When there's too much water, all the air is pushed out of the soil. This kills roots, since they need atmospheric oxygen to live. Drainage is not puddles on the surface in this case, but excess water in the soil, in spaces where air should be.