A very strange combo box behaviour and very difficult question

I have a combo box with customers. By selecting the customer i want to find out all the sales for this customer per months.If i select a customer the report shows only the first sales.But if i write down the name of the customer and not select it, then i receive the right figures.I am puzzled.
In order to get help i will have to explain myself more fully. I have a combo box for choosing the customer and then open report. The report summarizes the sales for the chosen customer
per invoice date - for months. The summed liters are placed in the InvoiceDateFooter, not in the details section.May be this is the source of my errors.May be the reason for my mistakes is the lack of a detail section in the report. Now, if i choose a customer from the drop down list of the combo box, then i open the report and the result is wrong- it shows only the first sales for this customer and not the all sales.However, if i write the name of the customer,find a customer and press Enter, then the report shows the right fugure !!!!!! The name of the combo box is Cbo1. Here is the code in the after date event:
DoCmd.OpenReport ReportName:="rptInvoicesPerCustomer", _
View:=acPreview, WhereCondition:="CustomerID = " & Forms!FSumUp![Cbo1]

I want to get the right figures not only when i write down the name in the combo box, but also when i choose the customer from the drop down list.
I will be very grateful if somebody help me