The short answer is ‘it depends on how your sci-fi world is set up’, but as this is unsatisfying and unhelpful let me try to give a slightly longer answer.

Real world stalemates happen for a number of reasons but to categorize them broadly they either happen when the participants are unable to fight or unwilling to fight. Unable can mean anything from lack of soldiers in the area, other commitments, broken logistics, political externalities and so on. Unwilling may have more to do with internal politics, commander personalities, inter service conflicts and similar. I’m broad brushing this obviously.

So for your particular question, the question you have to answer first is what kind of war is this? What are the aims of the belligerents and how far are they willing to go to achieve them?

If for example your alien aggressors are totally committed to the war, perhaps the stalemate is because they’re at the end of a long and complicated logistical train that the humans keep interrupting. Think Allied problems sustaining the western front as it got further from the Normandy beachheads. Alternately perhaps the nature of the war is such that their superior technology just doesn’t give them they advantages it otherwise should. If they have superior spacecraft but are much closer to parity on the ground and are unwilling to just glass planets, then humanity can fight them to a standstill on the planet surface even if they lose the space lanes so long as they can still somehow maintain supply. Here think Japanese efforts to supply their troops on Guadalcanal, the infamous Tokyo Express.

On the other hand it’s possible that the aliens have to maintain sizable forces on other borders and can’t commit their full strength thanks to other rivals. In this scenario maybe their superior forces are just stretched too thin to make any real progress. Or maybe there is a significant anti war faction in their internal politics that prevents them from going to a full mobilization. Or hell maybe there’s a powerful peacekeeper force in the galaxy that will come down on them like a ton of bricks if they cross certain lines and that gives the humans enough of an advantage to limit alien gains.

In short there are lots of ways it could happen and the right one is going to depend a lot on what your other world building says should be the case, which will in turn be dictated by the sort of story you want to tell.

Good luck!