After toying with the idea for years, last week I finally got a .22 pistol for target shooting, a Browning Buckmark. Feels and shoots great and I've been having a lot of fun with it. But the last two times I've been out, I've had problems with the slide not popping out after the magazine empties. It doesn't do it every time. It tends to happen most when I've been shooting for a while. I have kept the gun clean. Any ideas on what might be happening, or how to fix it?