Failure modes when flattening rings
When flattening rings for riveted maille, whether flattening the entire ring or just the overlap, there are two problems that the maille maker is likely to encounter.
Both problems are symptoms of the same condition - the ring ends are not lined up over one another.
If the overlapped ring ends are not situated directly above and below one another, when they are struck, to be crushed into one another, one of two things will happen. Either the ring ends will skip off of one another, or they will cross each other like an "X" and flatten incorrectly.
Here is what happens when the ring ends simply skip off of one another:
And here is what happens when the ring ends cross each other and only flatten where they cross:
If your ring ends line up well, then they will flatten nicely into one another, like this: