Does the Visual State of the buttons in the header change on MouseOver? We have that property IsHeaderHitTestVisible or something like that which is by default false and does not allow stuff in the header to get mouse events. It doesn't work in the last release but we have already fixed it and will be released in one of our internal builds.
If the buttons does not react on mouse over then I will be able to provide you workaround for the issue which will allow you to enable the buttons by setting IsHeaderHitTestVisible to True. In that case I will need to know which Silverlight version are you using (3 or 4) and which Telerik theme.
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