In kilnforming is it typical for any glass that touches shelf separator to be matte instead of glossy, because the glass picks up the surface qualities of the shelf separator, whether this is primer or paper.
The only kiln-formed glass that I can think of that is glossy on both sides are vessels made using the dropout molds. These are glossy on the inside and the outside, but even those will have a matte base if they are allowed to drop enough to touch the kiln shelf.
If you are practicing the flip and fire method, you can make sure the shelf side of the glass is very clean (sandblasting is helpful here), and fire it again with the matte side up, to fire polish it to an eggshell or glossy surface. However, at that point, the glossy side of the first firing would then placed against the kiln shelf, and that side will pick up a matte texture.
I hope this helps--if you have further questions about your results, please post a photo!