Not because you don't pressure your opponents enough, but because you press too much with a weak hand. Let's say you've got a pocket pair from 22-77, and you're facing a 3x the big blind raise from early position, and the raiser has 1/3 as many chips as you do. Let's also say that the raiser has 15x the big blind in their stack.

In all likelihood, if you reraise enough to put this person all-in, you're opening yourself up to one of two things - either you'll be putting a lot of chips in as a very marginal favourite (a small pocket pair versus any two larger, unpaired cards such as AQ or AK), or you'll be putting them in as a serious underdog (against your opponent's higher pocket pair, 88 ). In fact, only rarely will the best thing happen - very rarely will your opponent fold to your re-raise. The reason is that (1) they've committed many chips, and (2) they're likely on a decent holding given that they raised out of early position.

So, my suggestion is that you consider this - play your low pocket pairs less aggressively. They aren't usually going to warrant a re-raise, and usually you can't successfully bully people with them. Sure, you should open-raise to steal the blinds (like you would with any hand you want to play), but don't play these hands like you would QQ-AA or AK. Often, the best option is to call preflop when you can see the flop relatively cheaply, and then evaluate your chances on the flop. If you hit a set, you'll likely make some chips. And this way, when you miss and the board comes with overcards, you'll be able to save yourself some chips and fold.

My overall advice is this: if you're first to enter with a low pocket pair in middle or later position at a reasonably tight table (such as you'll find later on in an MTT, but not at the start), then you should raise to steal the blinds as usual (3x raise or so). If someone raises in front of you, you should either fold or call if you can do so cheaply from late position (with little chance of a re-raise behind you), with the emphasis being on folding if the raiser is very tight and/or is raising from early position. When you have a big stack, don't go hunting for the coinflip, and don't put your chips in as a dog. See the flop, and if you hit, then you can clean your opponents out.

PS - this advice holds true for cash games when everyone has a decent stack as well. Wait and see the flop if you can do it cheaply, and fold if you can't get on the cheap. And re-evaluate where you stand after the flop hits.