Telerik RadAjaxManager object return null on subsequent request

PageAjaxManager in second request .

It work well with first ajax request but in second request  $find("<%= PageAjaxManager.ClientID %>") return null.

This problem occurs browsers other than IE. In IE all ajax request are fired successfuly

Then set the EnablePageHeadUpdate property of PageAjaxManager as below and it will solve that issue. 

<telerik:RadAjaxManager ID="PageAjaxManager" runat="server" OnAjaxRequest="AjaxManager1_AjaxRequest" EnablePageHeadUpdate="false" EnableAJAX="True" UpdatePanelsRenderMode="Inline" >

Hope it may be helpful to resolve your issue.