Thanks Pavlina ,
I'm sorry I forgot to mention that I assign the default skin in the web.config
<add key="Telerik.Skin" value="Office2010Silver"/>
however i tried to do the below
<telerik:RadAjaxLoadingPanel ID="ralpAssign" runat="server" SkinID="Office2010Silver" />
but i got the same result.
in the first postback i get the panel appear but after that no.
it is working well in IE only i don't know why.
and if i use a page without masterpage it is working well in all browsers