I work in a field that heavily uses arxiv.org and posts papers before submitting them to journals, so it's common for people to send a citation request when they see a preprint that they think has overlooked their work. I do this pretty rarely, because I think it's often abused by people looking to pad their citation count by asking for citations from work that's only marginally related. Still, when I see work that very closely resembles something I've done, I generally send an email.
My question is what to do when a paper appears that is very closely related to my work, has not cited me, and it was clearly intentional. In this case one of the authors had a previous paper that followed up on--and cited--one of my papers, so I know they were aware of my work in the past. Furthermore, in their current paper they use a technical term that I introduced. There is no possible way that they could have written their current paper without being aware of its close relationship to my work, but they haven't cited my papers.
In this context, the usual email of the form "I have read your interesting new paper and wanted to make you aware of my related work" would be so disingenuous as to be ridiculous: they are clearly already aware of my related work. I can only conclude that they have intentionally chosen not to cite it. All of my encounters with these authors in the past have been friendly, at least from my perspective, so I can't imagine any interpersonal conflict that's behind the omission.
A similar thing happened to me once before when some authors wrote a paper that had enormous overlap with one of my papers, which they cited, and they corrected, in passing, a minor technical mistake that didn't change any significant conclusions. Subsequently they built a minor industry on this work but never again referred to my previous work, with the result that their papers that were only a very minor improvement on mine have been cited several hundred times more than my paper has. So I may be overly touchy about this sort of possibility.
At the moment I'm inclined to do nothing, because any message I could imagine sending them would come across as either disingenuous or combative. But I wonder if anyone has a suggestion for dealing with this situation.