Dual cable 904 sticking in Park

I rebuilt my 904 several months ago and it's been working great. Recently (6-7 times) if I put it in park on even the slightest hill, It will shift into park but the parking detent will not disengage. The only way I can get it out of park is to rock the car back and forth while I shift from Park to N or R. My cables seem to be adjusted properly so not sure why this is happening.