Is the form based on a single table or query?
If the record source of the form is spanned over multiple tables, any blank fields within the form could disallow the referential intergrity of any relationships to save the data. If this is the case, to test this theory, at the point where you cannot navigate records, press the escape button. Although this will undo any data entered, if it allows you to continue, it is likely that the integrity of a relationship is being breached.
See how you get on...