We recently printed with glitter inks and were informed that the print peeled off several of the shirts. My suspicion is that we did not cure it sufficiently, as it went on quite thick! Are there any guidelines as far as the metallics - diff. temp, for example? Other than that one issue, they looked great .
I have found that with glitter and metallic inks, it takes longer to get up to cure temperature; these inks tend to reflect heat, so I slow down my conveyor. As long as they get up to temp (320 for the ink I use), they should be fine.
Flash unit was all I had for my first year. Try raising the heating element a little bit, so it takes longer to get up to temp; then you won't risk burning it, but it will have a chance to heat through the thicker layer before your temp gun or strips show it is up to temp.