I think we all need a bit more specificity. What sort of piece is
this? Ring, brooch, necklace, pendant, earrings… What does the
snapping together have to do with anything? Is it just to avoid
soldering, or is it some sort of catch?
If you’re thinking of a magnetic catch you’ll want to use rare earth
magnets, also known as neodymium magnets. K&J Magnetics carries a
wide variety and sells in all quantities, so you can buy a few in
different sizes to see what works best.
As to why your ball catch doesn’t hold, cast metal is not especially
elastic. Once it has been stretched by pushing the ball in it will
not regain its original shape to capture the ball. Also, because of
the metal’s shrinkage on cooling you’re not likely to get a close
enough fit right out of the mold anyway. For these kind of catches to
work the tolerances must be very close and it’s best to cast over-
and under-size and then file or machine to fit. But really you want
the catch parts to be worked metal, not cast.