Please note that when using Batch editing mode for RadGrid all changes are performed on the client and are submitted to the data source with a single request after the user clicks on the Save Changes button. In case postback is performed before the modifications are submitted, they would be lost. In the scenario you describe such postback is performed after every row click. Because of this changes are not applied.
If you would like to save the changes when a row in the MasterTable is clicked you could disable the EnablePostBackOnRowClick property and handle the client-side RowClick event. In the handler you could call the saveChanges() method for the modified DetailTable.
If you need more information on the client-side GridBatchEditing API it is available in this article
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