Uchi mata is NOT that difficult a throw. It is certainly not the hardest throw to master. I find that thought a little silly...
Your problem is two fold:
1) You're fighting people higher ranked than you, and that just makes life difficult.
2) You're trying to attack a leg.
You shouldn't be attacking any leg, unless you're attempting a ken-ken type uchi mata. You should - in a regular uchi mata - be trying to split their legs right up the middle. The actual lifting action comes from the hips and is generated by the raising of your leg. That leg, however, should be straight up the middle of your opponent's legs, and it shouldn't really touch anything along the way.
This is much easier to demonstrate than it is to describe in text.
Now this isn't to say that you CAN'T throw uchi mata by attacking someone's leg. You certainly can, and there are plenty of people who are able to pull it off, but in general, you'll have an easier time if you get that idea out of your head. Drew Foster alludes to this concept in his post above, by suggesting you "attack" the opposite side leg, and it is sound advice.
There's a great competition video of what I am saying... I'll try to find it.