I find that when there is a lot of stuff floating around, that's when mine comes out. So if I'm fragging or blowing detritus off rock, in a few minutes it'll open up. Also it will open part way through feeding, usually full open after the food is all gone. That's when you target feed them.
Also they usually open up right after lights off. Contrary to the name, they do NOT like light. So if they are in direct light, that's part of the problem. Mine's at the bottom of the tank, in front, with a large rock wall blocking the light
Look really close, it's about 1/3 of the way from the lower right corner of the pic.
Also, the more you feed them, the more they'll stay open. I've seen them placed out in the light and opened up in an LFS
but he feeds them 5x a day.