These are pretty common problems for dudes who get to the jiu-claw (Eddie Bravo's loose omoplata sort-of set up) and can't finish the omoplata. I'll walk through the whole technique once you've gotten to that shoulder isolation position (there are so many ways to get there that it's really not worth going over even the basic ones in a single post, I'll do that later).
Well, I cant ever seem to connect a omoplata. Either my leg gets stuck under his arms, or he rolls out even if im holding him around his waist with my other arm.
The first thing that alot of people forget to do, and you may not, but it's something that I always remind people of, is to wrap that arm into your hip so that you have control of it. This is where the shoulder lock comes from and, without the arm trapped, your opponent actually has the better position.
Once you do that, you should grab the belt, if you need to hoist yourself up. From this position there are two ways to finish the omoplata, one is mine and the other is the traditional way, they're both simple, so I'll walk through both of them.
This basically puts you into the position of a bottom side carni with no hands.
I'm going to keep that arm trapped with one hand and climb up the back with the other. I'm not skooting my hips out (which can sometimes make this harder to do, but doesn't finish the omo the way that I do it) and I'm looking for a half nelson on the opposite side.
Once I put my arm under my opponents arm and get the half nelson, I'm going to finish by pulling myself up and cranking in the omoplata. It should look like a wierd crucifix with you on top of your opponent instead of having them flipped over.
The Traditional Way
Basically, to prevent your opponent from providing muscular resistance, you are going to trap the hip (as you were saying), to keep them from rolling out and scoot your hips out to the side, dragging their arm and putting your opponent more on their stomach. Your legs should be facing away from your opponent and they should be opened.
From this position, just lean forward and finish the technique.
Anyway, to address your particular problems.
If your opponent is rolling through your grip, you need to work on your balance from that position. Drop your weight down onto the shoulder that's blocking his hips before you pull his arm out. Once you pull his arm out, in the traditional version, you've won, because he has no base to roll over on. If you want to try my version, then just keep your upper-body weight blocking his hips, you don't need it to finish the omo as you climb up his body, the torque in his shoulder from the movement is sufficient.
If your legs get stuck under you when you're trying for a traditional omo, then transition to my omo. That's one of the reasons why I started using it, because alot of guys will grab my feet and keep me from getting them outside to finish the traditional version. Just climb the back and make sure that their arm is tucked in your hip, this'll finish them.