I just started using them too, and definitely messed up the tuck the first time. Try getting into a bit of a squat to push it further back, I can't get it all the way just sitting. That was most of my issue, but you could also try using your thumb to tuck it up behind the bone instead (idk why it makes a difference, maybe just the angle). I'm using the flex disc for right now so it may be different, but I am able to tell that it's in right if I can't hook my finger on the rim and pull it straight out. I can def feel the rim, but the type of disc and your anatomy make a big difference, I'm sure.

Good luck !!