So as you describe the technique, it seems that you all production steps make well. Here possible sources of error you can check.
Are alle enamels the same brand?
Are all your enamels: Medium Temperature - Medium Expansion enamel for copper, silver and gold?
Did you high melting enamel over soft melting enamel melteing or contrariwise ?
Did you anneal the metal before enamelling?
If it is possible, dome your workpiece a bit. That can cracking prevent.
Lead bearing over or under lead free? You can become aware it:
Unleaded
UNDER leaded.
Edmund