Hi Quang,
We are not sure what my be the reason for experiencing the problem. Do you receive any javascript errors? If javascript errors occur on the page they may confuse the proper functioning of the RadUpload control. Please, make sure that you have added a ScriptManager in your aspx page and the ASP.NET AJAX Configuration Elements in your Web.config file.
ASP.NET Ajax Framework on which all Telerik's Prometheus controls are based needs some additional settings in the Web.config file more detailed information about which you can find
here. For your convenience I have also attached a sample Web.config file.
In case the above suggestion cannot help you solve he problem, please, send us your application so that we can test it locally, research the problem and do our best to help you.
Best regards,
Sophy
the Telerik team