Make sure that there is a reference to the corresponding assembly on the page
<%@ Register assembly="Telerik.Web.UI" namespace="Telerik.Web.UI" tagprefix="telerik" %>
or in your web.config file
<add tagPrefix="telerik" namespace="Telerik.Web.UI" assembly="Telerik.Web.UI" />
If you still experience problems after that, please send us your project or a sample one where these problems can be reproduced so we can investigate further.
All the best,
the Telerik team
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