Donna,
Your slide stop should not do this. One of two things is occuring. Either it does not engage the slide enough, or the angle of the interface between slide and slide stop has to much of an angle.
If it does not engage the slide stop enough, figure out why. I could be the magazine, the magazine follower/slide stop interface, or a burr on the slide or slide stop.
If the angle between the stop and slide is to much, then the quick fix is a new slide stop. Especially if you have an extended type. There are $12 slide stops available, then there are
good ones available. I would advise a good one, as a Colt Series 70 is a good gun, and the slide stop is a part that gets peened easily. If you put junk parts in a nice gun, you are making a junk gun. A gunsmith could adjust the angle of the existing slide stop, but chances are the job would cost about the same as a Brown replacement. The replacement may or may not need fitting. Buy quality though.
If the slide notch itself is worn down, Uncle Nick's solution is the way to go. I'm betting that is not the case though. Since your gun is a custom job, I'm betting the builder used a cheapo MIM slide stop that has gotten worn down over time. Replace it, and you will likely be in business.
I would not try to cure this problem by carving a detent recess in the stop. I would only carve a recess to hold the slide stop down, not to make it go up. If you need the part to go up, figure out what is preventing it. Correct that. To many people have solutions before they see the problem.