I can think of five broad reasons for not citing someone:
These may take on a variety of concrete forms, and are, in some cases, interconnected. They are more or less justified, and more or less nice. (Aesthetics feels different from the rest. What I mean by this is that a slide, for example, should only contain a small amount of text, and that that text should not be a bunch of obtrusive references. In this case, appropriate citations are ideally provided, but somewhere out of the way.)
I try to be a good judge of relevance and to avoid sweeping under the rug existing work that could threaten the originality of mine. But I am sometimes victim of laziness and, always, of a decent amount of unawareness.
Whatever the reason that someone's work ends up not being cited when they think it should have been, it's reasonable to expect that they will feel cheated. I know I have felt that way. And it's not always easy to tell someone that they should have cited you. This is all to say that if you feel like I should have cited you, this is a free pass to please let me know.