Metal solidifies first wherever it is thin and then draws from
wherever it is thick as it cools. Sterling and 14K are forgiving
with respect to this rule, but fine silver will not let you slide.
Unless you provide enough feed for the thick parts, that
solidification will cause the juncture (from thick to thin) to look
like a wet sponge, or it will look like just the surface of the metal
has ripped open. Is that what it looks like?
As a rule of thumb, when you think you've over-sprued it, that might
be enough. You can try using a larger center sprue, also, that might
help. We use the lowest flask temperature possible (just above
getting no-fills) and the lowest metal temperature possible. We do
use larger buttons. Reusing clean fine silver doesn't seem to
present a problem.
Give yourself a break, though, if you are having trouble casting
fine silver. It is a difficult metal.