This snippet seems to be working fine with me as you can see in the attached video. You can also compare your page with my sample that I am also attaching to find the major differences.
Generally, when adding a target control you would need to provide a mechanism for loading the content. You can, for example pass the wanted text to the Value of the tooltip and in OnClientSHow obtain it:
You may also want to take a look at the Autotooltipify
feature of the RadToolTipManager.
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