A couple of things tiny bubbles in enamels can be from,

First the addition of Klye-fire.  I do my best not to use this but I realize in some designs we need it. The last time I used Klyre-fire, a mixture of 1-5 parts distilled water was best. It helps on a 3d surface but I could still see a few tiny bubbles. Klyre-Fire is very helpful on plique a jour. But you do see time bubbles.

Second way you can get those frustrating tiny Bubbles in enamels can be from sterling silver that has not been depleted properly. The more you fire the worse they get.*(

And last that I have seen, specks that 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.