[AccessD] Write Conflict

Firstly, I would like to wish everyone on the a Happy and Prosperous 2006. 
May the best of 2005 be the worst of 2006.

My problem is I am working on a video library database. My hire form has a 
main form (hirer's details) and a subform (hiring details). There is a 
combo box that the user uses to select the movie. If they double-click on 
the combo box they are taken to a new form where they can change the rental 
type for the movie (eg is it an overnight, 3 nights, weekly etc rental) 
When they have selected the new rental type the program checks for every 
instance of that movie name and changes it to the new rental type. It also 
updates the Hiring record to record the new date due into the table. This 
all works perfectly. However, when they close the form to go back to the 
Hiring form, I get Write Conflict. If I click on the Save option it works 
perfectly and updates the row in the Hire form details to reflect the 
changes and I can continue to work.

I can't seem to figure out where the error is being triggered so I can tell 
it to save programatically and leave the user none the wiser.

Any help in this will be greatly appreciated.

TIA

David