Too fast (hot) would be my guess. I don't do a lot of big dropouts, but I've done 50 or so small ones. I usually just make squares, 4.5 inches on the small mold Bullseye sells. Seems to me the hole is 3 inches. And I do tall ones, 8 and 9 inches of drop. Depending on how much effort I put into the design, I ramp it up to 1250 or so and hold it there for around 6 hours to get it to drop that far.
I also put a fiber paper ring on the mold, just because it gives a bit more grip for the glass. I cut a set of them and reuse them until they tear. But with a 4.5 inch square, I have around .75 inches in some spots, and no more than 2 inches anywhere. You get really bored waiting for them to fall, but the downside of not waiting is the failure you're experiencing. So if you're following the schedule, perhaps your kiln runs hot and you need to dial down the hold temp.
It will sometimes, with opalescents, take an hour before you see much movement, and 3 hours before you see an inch of movement. A bigger ring should move incrementally faster, but this isn't a 30 minute slump.