This troubleshooting help article covers some solutions for when you have add an AjaxSetting for a control that you want to AJAX enable.
If you have added AjaxSetting for a control to be ajaxified, but it still makes regular postbacks you can try one of the following suggestions.
If Your AjaxSettings Are Defined in ASPX You should verify whether the control ID matches completely in the corresponding AjaxSetting. Also,you should check whether the RadAjaxManager designer shows your control as ajaxified, which will ensure that the manager will find and ajaxifythe control at runtime.
If Your AjaxSettings Are Defined in the Code-behindCheck if you are adding your AjaxSettings on any control's event-handler? AjaxSettings are not preserved in the ViewState so you have to add them on each PageLoad. In cases like this, you may find your "ajaxified" control making a postback on the first try.