Have you ever been reading a scientific paper and thought "Gee, they really should have cited us here"?
Anyway, the question of the day is what you do about it?
As one wag* on Twitter put it, you could harbor resentment over the slight for next 30 years or so. Naturally, you should transmit your hatred for that other lab to your trainees well into the future. It goes without saying that you will subsequently attack all of that lab's grants and papers, and their academic progeny's grants and papers, when given the slightest opportunity to do so.
You might also just ignore it and let life work itself out. The rest of the field will also have the same thought if your work is actually related to the extent you assume, correct? And what is a citation here and there in the long game, eh?
Oooorrrrrr.... You could send a passive-aggressive**, or just plain aggressive***, email with your relevant papers attached.
What do you prefer, Dear Reader? How has it worked out for you? Is there ever any noticeable result? Other than venting your pent-up spleen?
Do you imagine these scenarios result from genuine ignorance or did the authors have an intent to conveniently fail to cite you? Are there mitigating circumstances of any kind? Is it possible that while you consider your work obviously and necessarily related, that this may not actually be the case?
**Congrats on your incredible paper, really great to see such convergence with our work (attached).
***You are failing to give my postdocs their due you scumbag!