This sounds like a Slide Lock Failure. On the last round the slide should lock open. Could be several causes. The slide lick itself, but most probably a dirty mag. If the mag is clean, then could be a weak mag spring.
6 or 7 shot mag? Most likely caused by weak mag. spring or the follower.
For best operation,put in flat follower with dimple and a 11# Wolf spring.
Doing this cured all my problems with my 5 mags.(both 6 and 7 shot)
They are all 6 shot now and work great.