Black metallic spots on fused glass backs

what is causing ugly black metallic spots on the back side of fused glass.  Been teaching this for 10 years and never saw it before.  Tried everything.  Only thing different is using Wissmach glass (ran out of Spectrum).   Maybe something new added to our glue?  Alene's quick dry.  Anyone else having a problem?  It has happened about 5-6 times.  I could post a photo, but don't see a place to do it.