Could you go through this help article and see if the requirements there are fulfilled: http://www.telerik.com/help/aspnet-ajax/tooltip-troubleshooting-common-issues.html
? Most importantly - the tooltips need dimensions.
Also, you can try setting the ContentScrolling to None or Default if it is Auto to see if this changes the behavior on your end. There is a known issue that you can examine here
, together with another workaround.
Another possibility is that the content received is larger than the tooltip, so the browser stretches the container element. In this case the width would need to be increased.
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