UMP bolt doesn't always stay locked back
 

after last shot is fired, it locks back.

when you insert an empty mag and charge the handle, the bolt locks back.

but if I charge the cocking handle under control (very slowly) past the locking point when it's locked back already with empty mag, sometimes the bolt goes forward.

the make is VFC UMP

bolt catch spring issues? Maybe the catch is rounded?

Has anyone even worked on a VFC UMP yet? lol