sum on subform and main form

Sorry that this is rather basic. But I have tried the various
suggestions given in this group and cannot get them to work.

I have a main form, for simplicity called "Main". On it are two
subforms. For simplicity subform1 and subform2. subform1 lists fees
for items. The sum of all fees for all items is in a text box on the
footer of subform1. PurSum =Sum([Fee]). subform2 contains exactly
the same field, fees for items of a different type. The sum is again
given in the footer in a text box, SaleSum = Sum([Fee]). So far so
good and it all works well.

Try as I might I cannot get a text box in the footer of the main form
to display the total for the total boxes on each subform, ie Total =
PurSum + SaleSum.

Have tried Total = [subform1].[mainform]![PurSum] + [subform2].
[mainform]![SaleSum] to no avail.

Any suggestions or observations would be gratefully received.

Sorry to all the experts to whom this is an elementary point fully
covered elsewhere in the group. Have looked for three hours and tried
different approaches without success.

.