I wouldn't worry too much about not levelling up, thanks to the strange levelling system the game uses it's quite possible, and it doesn't neccesarily mean you've not been getting your skills up either.
Basically, your character levels up every time you improve all your major skills by a certain amount, while improving your minor skills will do nothing to your level. Which means that if your athletics, or blade, or sneak, or light/heavy armor are in your major skills, you'll likely be levelling up fairly quickly (as those are the most used skills), whereas if they aren't (and your major skills are set to ones you rarely use) you'll instead take quite a long time to level up.
Now this system itself I don't have a problem with really, it's the fact that the game auto-levels itself to your level as well that annoys me. One or the other would be fine, but both together creates some annoying gameplay for me. My first character in the game, I'd spent about 15 hours or so playing with, and had rushed fairly quickly up to level 12 with, but the game just became more and more difficult for me, until I couldn't even beat the most basic enemies with my strongest weapons without a massive long battle with. All because I had things like athletics, acrobat, sneak and speecraft as my major skills. Skills that I was using all the time, that I considered important, but that meant that while I was levelling up, I wasn't actually getting any stronger (as my blade/marksman skills weren't really being used as much as the others), but my enemies on the other hand, were (thanks to the auto-levlling system). Thus, the game got impossible, and I just had to start again and rejig my major skills to, stupidily, skills that I actually use less.
Nice idea for a system, but quite flawed. So just because someone is on a higher level than you doesn't neccesarily mean they've been playing more than you. Or that they're stronger than you.