I'll share some information to ensure that the beginning balance for your next month's reconciliation is correct.
These are the following reasons why you'd encounter ending balance issues:
At the start of the reconciliation, someone entered an incorrect ending balance.
There are missing or duplicate transactions in QuickBooks.
Someone entered transactions into QuickBooks that have not yet been cleared by your bank.
To resolve this, you'll need to undo the entire reconciliation with the assistance of an accountant who has a QuickBooks Online Accountant (QBOA) account. You can use this link to find one near you: Find a QuickBooks Certified ProAdvisor.
Consider reading the following articles to learn more about the process: