As of Q2 2015 SP1 the popup element of RadToolTip cannot be shown if the content is empty. This is a result of an important improvement—http://feedback.telerik.com/Project/108/Feedback/Details/54180.
This change will affect implementation where content of tooltip is populated via the OnClientShow event. The proper usage is handling the OnClientBeforeShow event in order for the tooltip to show up.
You can also examine this online demo (http://demos.telerik.com/aspnet-ajax/tooltip/examples/databasetooltipswithoutlod/defaultcs.aspx) to further see such an implementation.