Forms :: Form Won't Calculate Totals

I am having trouble on the Work Orders Form to get the SubTotal to calculate correctly.

The SubTotal Control Source is:

Code:

=DLookUp("[Services Total]","[Services Total]","[WOrderID] = '" & [txtWOrderID] & "'")+DLookUp("[Parts Total]","[Parts Total]","[WOrderID] = '" & [txtWOrderID] & "'")

The Form Record Source is:

Code:
SELECT DISTINCT [Work Order].*, [Payment Total].[Payment Total], [Services Total].[Services Total], [Parts Total].[Parts Total] FROM (([Work Order] LEFT JOIN [Parts Total] ON [Work Order].WOrderID = [Parts Total].WOrderID) LEFT JOIN [Payment Total] ON [Work Order].WOrderID = [Payment Total].WOrderID) LEFT JOIN [Services Total] ON [Work Order].WOrderID = [Services Total].WOrderID;

why my form won't calculate totals?