What are the general problems that people face getting out of the mount position?
Just like any position, you have to consider skill as well as size and strength.
You say he's 22lbs heavier, but if he's less skilled, then your technique should be overcoming that difference.
If he's slightly less skilled or up, then you will need to do something to overcome that difference because it will make his weight advantage matter even more.
To do a proper upa escape you want to trap the leg to the side you're rolling to, but you also need to capture his arm on that side as well and hug it tight so he can't post out. Then you need to strongly bridge and roll to that side. The tighter he is to your body the easier it will be. If he is able to rise up and get space between your bodies, the physics will be against you rolling him.
If you're around the same skill level and you're sure your technique is correct then you need to take it to the next level and start putting your mount escapes into combination. If the upa doesn't work, go to elbow-knee, foot lift or foot drag as appropriate. You have to use upa to set up the other escapes and vice versa.
In intermediate levels of grappling, people gain a sense of balance and defense from bridging; either they have their knees out and feet tucked in nicely, or you can't seem to secure a side.
The next time you get stuck, make a mental note to check your position in relation to your sparring partner. Some things to consider are:
1. Where are his feet? Are they tucked deep into your sides? If so, your "upa" will not be as effective. You need to get his feet back out through good positioning. Your best defense is to get to a workable posture before this happens.
2. Don't rely on isolated strength, such as your arms, legs, etc... Use your entire body. If you must develop strength, develop core strength. Right now, you need technique more than strength. Strength is only a temporary solution. Just imagine if you figured this out without it; how much more effective it will become when you do become stronger.
3. Develop a system of escaping from the mount. For instance, many people use the upa-to-elbow escape method. They attempt one, then move to the other. In MMA this is combination is less common, but you get my drift.
Obstacles and challenges are a part of training. It's how you approach and overcome them that matters. The key is not to give up.