What could be causing the big discrepancy between Checking acct balance in QB and the actual bank balance?

All the transactions are not posted to QB, assuming the bank is correct.

Please explain. Is there a report showing the transactions not posted?

Are you performing the reconciliation within Quickbooks? Enter the bank statement balance, then mark off in quickbooks in the reconcilation that cleared. If there are any bank entries you didn't make in QB (for items set to autopay with your bank for example), make sure to create entries for those. Should reconcile and report is produced when you are finished.