Can anyone tell me what causes bubbles in my glaze?

I am a ceramics newbie and my first glaze left lots of bubbles on some of the pieces.
I fired 25 identical (for the most part) pieces and the pieces varied from zero bubbles to one piece with a dozen or so.
Could it be that I didn't fire the bisque hot enough? Or is the temperature for the glaze all wrong. The bottle said to fire the glaze to cone 06 but all I had was 07 cones. I also fired the bisque to 07 (but I do have 03 cones for that)
Using a kiln sitter so the cones are sitter cones.
I don't have any witness cones.

P.S. This was my very first time with clay and glaze so I'm not too surprised that I am having problems.

Also, is there a more active ceramics forum out there somewhere?

Thanks