Double cast sterling and gold

Help, please! I tried to double cast sterling and gold with mixed
results and I hope someone can giveme some direction… I first cast a
sterling band in the lost wax method. A wax design was created around
the sterling band and then the assembly was re-invested and cast in
gold. The resulting rings came out very well. The problem is that
when I made a third ring with this method the casting didn’t turn out
quite the way I wanted. The ring was cut up to salvage the metal for
another try. The problem is that the sterling had a void inside and
had puffed up like a pastry. It also looked as though the copper had
seperated from the sterling. Any suggestions would be appreciated.

Regards, Paul Matheson