Pivot table problem

[ReportSmith]

I have a query as the source of a pivot table (form). The first time I
started putting fields onto the form (in pivot table view), it worked great.
I made some changes in the underlying query and am resetting up my pivot
table...every time I put the amount field in the detail field area, Access
displays a "Preparing Data" message in the status bar and everything locks up.

I've come across this before and my solution was to recreate the pivot table
- it worked...but this time, it doesn't. Does anyone have any pointers?

Thanks in advance.