Found published work that doesn't reference previous related work
Realistically, there is nothing you can do to change an already published paper. There are some options for extreme cases, but they are unlikely to apply.

What you can do is try to have the record correct in future publications. Unless you have hard evidence to the contrary (and usually even then), it is best to communicate on the basis that this is not an act of malice. You can write the authors of these papers (and authors of preprints on the topic) a polite email stating that you liked their interesting work and have done related work that you think might be of interest to them. What they do with that information is up to them then.