A couple of things bubbles can be from,
The addition of Klye-fire. And some times the more firings the more they go crazy. I do my best not to use this but I realize in some designs we need it. The last time I used Klyr-fire, a mixture of 1-20 parts distilled water was best. It help on a 3d surface but I could still see a few tiny bubbles.
Bubbles can come from sterling silver that has not been depleted properly. The more you fire the worse they get.
Specks usually come from old or deteriorating enamels. When you wash them you see tiny white specks in the container. On your first few firing you will see some are white in your enamel, then continue firing some look like beige-brown goo, and sometimes they turn into bubbles.