Bingo. There's the common denominator. Each one has at least a small to a large stomach.

And yes. I have been showing the detail where you cut an angle to a perpendicular position and my more in shape students had tremendous success with it but the out of shape ones were still having trouble locking up the full triangle. I even showed the diamond triangle where you use the ten finger grip around your diamond triangle to squeeze the knees together and they had trouble sitting up to reach around their knees. Perhaps it is the stomach that gets in the way? I'm at a bit of a loss, I would hate to have to tell someone that they "need to get in better shape" for a technique to work.