QB Issue Resolution:
Yes, you have the option to uncheck the transactions at once. This can be done by selecting all transactions, then deselect them.
First, be sure that all transactions are selected by clicking on the circle beside the Deposit column, then select Yes, select all to confirm.
To remove the check-mark for all of the transactions, click the circle again.
For additional reference about reconciliation in QuickBooks Online, you can read this article: How to Reconcile an Account in QuickBooks Online.
Resolution for Issue 'If my bank reconciliation doesn’t balance and I need to start over, is there a way to uncheck all of the transactions w/o unchecking each transaction separately?' available: Yes (Solved).
Source: Intuit Community forum.
If my bank reconciliation doesn’t balance and I need to start over, is there a way to uncheck all of the transactions w/o unchecking each transaction separately?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.Support for Banking
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