The first thing is does the button have VBA code or does it use a macro?
I am not sure how that code works on the subform when Diff1 is on the Mainform, because to refer to it should me.parent.Diff1, which was why I asked the question about where it was.
Have you tried just pasting your code in and add an Exit Sub after the "Diff1.SetFocus" ie before it actually goes to a new record.

ps your code should have error trapping in it to identify problems for you.