I don't know your kiln, but 20 minutes at 1500 degrees F seems a little long. For two layers of 3mm sheet glass, we usually choose a fusing temperature high enough to get the job done in 10 minutes. Reactions do increase with increased heat history (time spent in the fusing cycle in the heating, process, and cooling phases).
It is possible that your piece was fused with the non-irid side against the Lt. Cyan. The irid side is the side with the production stickers, and that could be characterized as the "dull' side. (Exception: on heavily textured glasses, if the label won't stick we might stick it to the back of the sheet, but it is usually on the front.)
The irid side also feels less smooth when you run a fingernail across it. I don't know how to describe this exactly, but when I am using a transparent irid glass, I look at the edge, and with the irid surface up on the cold sheet, I can see where the irid surface ends and that this is above the cut, non-irid edge.
The "dull" side is the side with the irid, which acts as a reaction barrier. This needs to be against the cyan sheet.
If I were you, here is what I would try. Make a 4" square test tile. Make sure the irid surface is against the cyan surface. Same cycle, except shorten the process temperature to 10 minutes at 1500 degrees F. At the 10 minute mark, I would open the kiln door an inch or two to peek to see the piece was fused enough. If not, I would add time in 5 minute increments before continuing the to the cooling/annealing phases.
Can you try this and report back? Good luck!