Updating value in one subform cancels out update in other subform

[skips84s]

I have a parent form with four tabbed subforms. On subform x, selecting a value from a combo box should update a few controls on this subform, as well as a control on subform y, all done using dlookups. Each of the controls does have a valid control source in the same table. Right now, I can update the controls on subform x with no problem. When I add the code to update subform y, however, the control in subform y updates and saves the value, but the updates made in subform x are not saved when move away from and return to the subform. Here is how my code looks:

Private sub cboControl_AfterUpdate():

control = Dlookup(...)
Me.Parent.subformy!control = Dlookup(...)

End sub

I appreciate any help with this, because it's driving me nuts. Thanks in advance,
Stephen