My guess is the mech is dirty or the plastic lock piece is worn/broken. I had this issue and had to take the whole mech apart and clean it. Ended up replacing the plastic lock piece as well. Swinks makes a repo part:
https://www.shapeways.com/product/ECLX7GYN2/03-8090-flat-cam?optionId=97793645&li=marketplace
I'd take it apart and clean it first.