I may be wrong, but it sounds like you may be using glitter rather than mica. Mica has a very high melting temperature. The coloured mica used for kiln forming is formulated for higher temperatures than coloured mica for glitter.
The only “problem” that I have with mica powders is that only a thin layer sticks to the glass, so I usually use borax with the mica to soften the surface of the glass a bit. If I put a lot of mica between glass, I get lots of large bubbles.
So a solution for your material may be to fire twice – once to full fuse, second time add mica and take to a tack fuse.