Robert -
Not enough detail to help diagnose the problem.
Please post some screenshots of forms (in design mode) and the code behind field to help us out.

Pic No. 01 - MainForm = SourceDocumentF - Total is the field  that retrieves Dsum of a value from subform LocationTF
Pic No. 02 - Subform = LocationTF - Shows the field "QuantityRpt" where the field from the mainform obtains its values.  There may be several records represented by the total
Pic No. 03 shows the Main form after the subform is closed and saved.  The value in the textbox does not show.  The query at the bottom does.
Pic No. 04.  The field populated with the value of 273 after I click on it.  Once I save the form after I click on it, it will save in the table.
Pic No. 05 - Table SourceDocumentT where the value is stored.
Pic No. 06 - VBA Code used to get the value from the subform to the mainform

Robert, something has to trigger that value being saved in the table. It looks like that something is in the Click event of a field. Move that into whatever other Event you want to run that event. Since you haven't taken any of my VBA classes (Developer Series) I don't know what your skill level is, but I'd recommend you at least watch my free Intro to VBA video.

Thanks Richard