I can't generate a 90 day statement for a particular client even though I change the date parameters
Welcome and thanks for posting in the Community, @malcolm8.
If the dates are already correct, the browser's cache and cookies may be causing this odd behavior. In this case, try signing into your QuickBooks Online (QBO) using a private browser (incognito).
Press Ctrl + Shift + N (Google Chrome).
Ctrl + Shift + P (Firefox).
Control + Option + P (Safari).
Once logged in, try to generate your client's bank statement again to double-check. If it's successful, return to your default browser and perform a clear cache to refresh the system. However, if the issue persists, try using other supported browsers.