Assuming that the transactions for the month that you're reconciling agree exactly between Quicken and the statement, then it seems like it almost has to be that some transaction or transactions reconciled in previous periods have somehow changed. One not infrequent problem that's been reported regarding Accounts set up for downloading is that a download changes the opening balance of the Account, so that's probably the first place to look.
If that's not the problem here then you'll have to work backwards throgh the prior months to see if those months still reconcile.
