A few months ago I bought these two different sizes of rivets for some strapwork I do, and I'm having trouble getting them to hold:

1). RV664 Double Cap Rivet, Cap 6mm, Post 6.4mm

2). RV679 Double Cap Rivet, Cap 6mm, Post 7.9mm

Two out of three of them, after I smack the cap down, I can easily pry it apart with my fingers, which is annoying. I note that they both have the same size caps, does that sound right? The reason I ask that is that it seems really hard to get the caps to snap down on the posts with my finger like they should, before I use the mallet to smack them. If they're right, is there a trick to getting them to snap? I also have a smaller size (both posts and caps) and they snap easily.

Another possibility is that I only have one 4.7mm rivet setter. I know it's too small for the above rivets because it doesn't smack the caps nice and even, but it might just be cosmetic. Is this why the rivets aren't being properly set?