I'll have to agree with Rob that most problems with slide stops are related to the magazine. The follower may not be engaging the slide stop. Check to see if it's worn and if it's not, then your spring may be tired (you can buy the springs and followers direct from Beretta). Another area to look at is the hole in the magazine tube for the release.
If that doesn't work, then check the slide stop itself. Clean it well and see if any of the bluing is worn off anywhere; which may suggest that it is worn at one particular spot.
Concerning the gun locking back with one round still in the magazine, where are your thumbs? Be sure it's not you inadvertantly applying upwards pressure on the stop during recoil.