I also agree that it sounds like a 'Trust' issue.
A quirk (at least to me) of Access is that your File(s) can not be in a Trusted location, but if you open the File in Design View, do designing, or modifying, whatever, then run the Form, it'll execute all code.
But if close the Form, reopen it directly in Form View (i.e. click on it in the Nav Pane) all code will fail to execute! Apparently the Access Gnomes assume if you start out from Design View, you've the developer, and anything you do is to be 'trusted,' but not so if you open the Form directly!
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007