Drop down parameter selection

Hello,

I am fairly new to Reporting Services Report.

>> I have one Parameter to be entered in the report which is Invoice #

>> I have the second parameter Box which shows a list of numbers, which is related to that particular invoice number entered.

>> I put a default invoice number (I don't know if it is the right thing to do though)

>> For the first time I view the report and type in the parameter. The second parameter drop down list gets populated. (That is what I want)

>> Then when I try to enter another invoice number, it does'nt work, seems like the application hangs or something

Please could you help me with resolving this issue.

Thank you,