R.B.
I do the same thing..
If they are having problems with one problem then I have them skip it and move on then come back to it. I will try to help them, but some things I don't know ( I can't help them with math). If they still can't get it after going back and trying to find the answer or way to do it then I tell them to ask thier teacher in the morning. I usually put a sticky note on the paper saying remember to ask about question #XX. So that way they remember and the teacher knows we tried.
Unless they were goofing around and not focusing everything is done by dinner time. If they didn't get it then its for the teacher to help them I don't make them try after dinner. If they don't get it.. they don't get it. If they were goofing around then they have to work on it after dinner until its done.