We do not guarantee that RadAjax controls will work seamlessly with any other third party controls, so there are a few things to check before going with using both suites together. Please, review this help article and see if the things to check for are fulfilled for the controls you want to use with RadAjax:
Incompatible controls and updating 3rd party controls with Telerik RadAjax
If they seem to be compatible, please, post more information about your scenario - generally, what steps to take to create a project that replicates you problem.
the Telerik team