Based on the provided description it is rather difficult to determine what exactly is causing the issue. Could you please provide some more information on your scenario and test the approaches below for optimizing your project:
- Try using RadAjax for ajaxifying the controls and prevent flickering.
- You could set Shared Calendar for the RadDatePickers control in case more than one is used on the page.
- Please inspect the server resonance in the browser's console and verify if any js errors appear when the drop down list failed to open.
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