I would pull aside the one kid who had asked you for help and ask him what is going on. As for the others, I would just reiterate that you are available to meet with them and restate your office hours, but leave it up to them to initiate a meeting. It sounds like you have extended yourself a great deal already. However, if you feel you are talking above their heads or that they can’t understand the work, you could get in to a general discussion with the whole class about that.