[AccessD] Write Conflict

Dear List(s):
 
I have a form with a subform - both bound - to different tables -
parent/child - linked Main form PK to subform FK - standard stuff.
 
When I change the quantity of one of the fields of the subform, a quantity
field on the parent form gets updated. All looks good until I try to move
the main form to a new record.  Then I get the Write Conflict message - Save
Record, Copy to Clipboard, Drop Changes.  I can see that if I Drop Changes,
the quantity field on the Main form reverts to its original value.  If I
click Save Record, all is well.  But I need for that message to go away.
 
How to I work around this message?  
 
MTIA
 
Rocky