It's lame.
It happens so often here too. I can't count how many times I've been cheated out of a comment swap. And it's not like my comments are shit comments, I mean I really try and work on a good critique for you if I'm reading your story.
It's just really inconsiderate and then it puts you in an awkward place. Do I call you out on it or not? Because that can get edgy. You don't want to start a fight or seem annoying or get too angry over something as simple as a comment swap, but dammit it's irritating when you don't get your end of the deal!
So what do you do in this situation? Call out the person, sit back and wait to see if maybe they just haven't gotten to it yet, or ignore it completely.
Frankly I like to go the angry journal rant route. It's therapeutic, right?