I had a problem like this, although the override would always do the trick for me.

My theory was that the shiftlock solenoid in the "bucket" was sticky from years of grime built up in there.

I got some white lithium spray grease and shot it in the vicinity of the solenoid "pin", hoping it would soak into the sticky area and dissolve/lube whatever was causing the problem.
Not easy to see, so I really gooped it on.

After removing the shifter cover and doing this, I realized it would have been just as productive to push the spray lube tube through the bristles in the shifter channel and blast away.

Good luck.