Sometimes the inner snap ring will hang up - a little shot of silicon spray may free it up. Other than that, you may have to replace the snap.
Does the snap pop open "too easy" when the gun is not in the holster?
If not, it might be that the strap is too tight.
You can stretch the leather (it is a leather holster, right?) of the retention strap.
1 - use plastic wrap to cover your gun where the retention strap goes. A couple of layers
2 - moisten (not soak, not drench) the strap
3 - stretch the strap and snap it down
4 - put the holstered gun away for a day or two to allow the strap to take on it's new set
Just be glad it's not a one-way snap
http://www.romefast.com/SnapFastenersRoundSpringThreeEighthsInchOneWay.asp I'm not going to mention the name of the semi-custom holster maker who put one in that worked in the wrong direction. He was understanding when I called and explained that unless he had a fix I coud do in the next 30 seconds I was going to cut the retention strap so I could send just the holster back to him. The holster I got back was a bit fancier than what I had originally ordered as well as coming with a note of apology and assurance that he had triple checked the direction before putting the snap on this holster.
stay safe.