Kt17, I agree that manners matter, but a little reminder on the forum rules probably wouldn't change things. I'm not sure if many new Forum users even look at them. I don't think I've ever read them!
It is annoying to give an answer - especially if it's for a tricky clue - and to get no acknowledgement. I think most helpers/responders don't want undying gratitude, but it is nice to know that your answer has been seen, maybe even helped someone get on with the rest of a puzzle. If I'm in a stern frame of mind, I occasionally add to the thread, with 'I hope we helped?' or 'Did this answer work?' That sometimes results in an acknowledgment.
I know myself how easy it is to get engrossed in a crossword. Maybe these posters have used the given answer/s to battle on with their puzzle, and don't want to take even a minute or two away from it to acknowledge any help?