Wow...so sorry.

Was it a local charity / civic organization?

As a fundraiser myself that's shocking! Stewardship and cultivation of relationships is crucial and not even being thanked for any previous support you may have given, or even for attending in the first place is unheard of in my neck of the woods

It is probably just you, the way you look around you give off the vibe you want to be left alone. Try looking someone in the eye and smiling, that usually makes them curious enough to say hello. Another way is to stand in a group and just comment on the current topic. Don't let yourself be ignored.

Time to quit and move on!