Hi,
I seem to have an issue with my fretting hand pinky when stretching over to the 6th and 5th strings and applying pressure; the middle knuckle collapses so that the pinky is straight from the base knuckle through to the top knuckle (with the top knuckle bent), so that if you look at the pinky side on it looks like an 'L' shape. It means that my pull-offs become quite messy and weak.

Nothing I have tried seems to help keep that middle knuckle bent when stretching. Any suggestions?