Assuming you have used the inbuilt navigation forms, this is caused by your form being loaded as a subform of the navigation form.
The correct syntax you need will be something like
Code:
[Forms]![Navigation Form]![NavigationSubform].Form![YourFormName].[YourControlName]
This is one of the reason most of us don't use them. They add a level of complexity that isn't necessary.
A complete reference to forms and sub forms controls is here http://access.mvps.org/access/forms/frm0031.htm in case I'm a bit off with the syntax... (Quite possibly)