I have been researching a specific topic in computer science for a couple of years now and a well established professor and his students have recently published a couple of publications in that topic too. In their work they do reference the well known and well cited previous work that basically everyone in that topic references, but I have noticed that they ignore (don't reference) a couple of publications that are doing essentially what they are publishing about, i.e. extremely related work. And I am wondering even though there is a gap of a couple of years between the work they have published and the available previous literature how did they miss referencing the relevant previous work? Was it done intentionally or did they just do a hasty job at finding more recent related work?
Either way my main concern here is if there is anything that can be done to remedy the situation now, since the paper has already been published?
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.
What do you expect to see in references? Every article on the topic?
References should contain the articles that the article in question actually reference. They publish some results. If these results are in the area that those "well known" articles defined and set up - they reference those. If they use certain results found in other articles - they reference those. But they don't owe you a reference just because you do similar stuff in parallel. If your results weren't used in their work, there's nothing to reference.
If they are writing a survey, they should include every notable result and reference it. If this is the case, you could actually contact them and ask (in a non-aggressive manner) why your work hasn't been included. But everyday articles should just cite the articles that they base the work on.
If you suspect they simply don't know about your work, feel free to write something friendly like "hey, looks like we are working on the same stuff, I noticed that you found Fact1 about Topic1 in Article1, I managed to find Fact2 in Article2. do you think Fact3 could be true? have you managed to do any work on that yet?"