If the flipper stays extended with power off then it certainly seems like a mechanical issue. I would check the coil sleeve, flipper bushing, spring, and other flipper parts. The plunger should easily move through the coil. Best thing would be take it all apart, clean, and inspect all parts. If a lot of parts appear to be worn, do a flipper rebuild with new flipper bushing. If flipper is not gapped correctly, that could cause binding.