Calculated value not updated on right click filter

[vmon]

I have a sum calculated on a main form that is set from a text box in the footer of a subform. I have trained people using forms to use the right click and filter by selection for narrowing records in a datasheet. Problem is the calcualated value on main form is not being updated with the sum of the records in the datasheet. Is there a way to update this?

Thanks in advance,
vmon