I can totally relate too, they can be quite miserable to set properly.
Like Art said, it can be for several reasons.
the post is too long for the material, or too short.... they are the most likely reason...
but the other over looked is the tool should be held straight, not crooked, sometimes you can get away with a tool long post, if you just hold it straight up.
it maybe too the quality of material used in making the snaps... some are better than others, I try to avoid the junk sold from tandy, it mostly made in the orient.