Take a look inside either part of snap for a phillips head screw. If equipped, as some are, you can loosen this screw about 1/4 to 1/2 turn and adjust the snap's position. If there isn't a screw, it's possible you need to stretch the leather on the retention strap slightly.
Screw Adjustment:
1. Loosen 1/4-1/2 turn.
2. Insert an empty gun into the holster.
3. Fasten the strap by adjusting the snaps until they will close fully (all the way around).
4. Add a little tension to the strap by pushing the snap to "tighten" the strap. Mark the position of the adjustable snap.
5. Remove the gun, reposition the snap where marked and tighten the screw.
6. Check for fit. Readjust as necessary to get a secure fit.
Non-Adjustable snap:
Presuming you're getting the snap closed all the way, but at some point it "pops" open, here's a trick to slightly stretch the thumb strap.
Lay the holster in the sun for an hour or so until it's warm to the touch, especially the thumb strap. This will help soften the leather naturally. Insert the gun into the holster and snap the strap in place. Be sure the snap seats fully all the way around. Leave the gun and holster in the sun for 1-2 hours. Remove or shade the gun after that time and allow to cool with the gun in the holster. Once it cools, try it for function.
If it's still not quite seating, repeat the above procedure, but fold a business card in half both ways and insert this under the strap to stretch the strap slightly.