There's a kown bug in FileMaker that may be the cause here. Try using the slider in the status area to select the very first record in your layout and see if that makes a difference. You may want to include code with Go To Record/request/page [first] to select the first record in your found set each time you pull up a report on this layout.
For More Information see: Sometimes global field in header become disabled
This is one of many acknowledged bugs that can be found in the Known Bug List here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html
bummer... Unfortunately no luck with the go to first record, second, third, or nth for that matter. Actually the only thing that changes is after changing the record about five times, the field goes blank.
I guess my work around is going to be add a button that opens a small window in which you can change account and date...
Thanks for all your help