It sounds like they might not have used the correct 2-part seam sealer but just used the mastic for the whole thing, including the edges. I have installed a lot of this stuff in commercial/industrial locations, where there are many long seams because of the very large area covered. We always used the 2 part epoxy based seam sealer from Armstrong and have never had a problem. You need to really clean out the area very very well. Unfortunately, it sounds like you did not repair this immediately and let the lifted edge possibly get damaged or stretched. I would get the correct seam sealer and glue it down and it should be fine. You should rent a seam roller and maybe cover the seam after with wax paper and put a 2x4 over it with some weight on it to be sure that it stays flat and doesn't lift up during glue setting.