Likely spring bind. To confirm put a small dot of grease on the rubber end of the buffer, carefully reassamble without messing up the dot of grease. Pull the thing all the way back, and release, then disassemble and inspect. If the rubber tip hit the end of the buffer tube, the grease dot will end up being smeared. If the spring is binding on the coils, the rubber tip won't make it to the back of the tube and the dot will be somewhat intact.