I have a border with a variety of flowers planted. The border is eat facing and runs for about 50 feet. I have planted different plants over the years. Most of the plants that I plant in the second half of the border survive no more than a couple of months and then die. The only thing that is different between the first 25 feet and second half is the fact that the second 25 feet is under a very large sycamore tree. The branches are quite high on the tree, so I don’t think it is an issue with the shade, but I was wondering, is it possible that the tree is shedding something or could its roots be having an effect?
Sycamore trees aren't known for having allelopathic properties (releasing chemicals through their leaves, buds, nut hulls, and roots which are toxic to other plants), but their root system is incredibly invasive and could be creating a severe disturbance in the soil of that second 25 feet of your bed.
Read more about Sycamore trees here:
This article provides a list of plants that are more tolerant of allelopathic tendencies as well as those that are not tolerant at all: http://web.extension.illinois.edu/cfiv/homeowners/110811.html
This article will give you some additional tips about allelopathic plants and trees: