The provided information is not enough to replicate the issue. The OnItemCreated event of RadTreeList will fire when each of the TreeListDataItem client components is created. Note that the RadTreeList client TreeListDataItem objects will not be created until they are requested, for example by calling TreeListInstance.get_dataItems(). Until then, the OnItemCreated client event of the control will not fire. Please have a look into this online demo
for knowing the working of client side events of RdaTreeList.