Cast heads in place on a ring

I’m trying to cast heads in place on a ring- I have a commission for
a wedding band in gold, a wax with several small heads. I’ve done two
practice pieces in silver. The first time the heads didn’t stay in
place and were lost. The second try, the heads looked great, but when
I set the stones, the prongs were very brittle and broke easily.

Why did this happen? Did they get too hot in the casting? Is this
more likely to happen in silver, and would I have the same problem in
the 14k?

Janet Kofoed