RadFormDecorator's purpose is to decorate a list of predefined simple HTML elements as well as ASP controls. You can find that list in the Integration With Standard Controls
Could you please provide more information about the controls which are not decorated, as well as to what is the actual and expected behavior? If you mean the RadInput control, then there is a know issue in Q2 2013, so that when a decorated RadInput is hovered, it looses its decoration. More information about the issue as well as a workaround about it is available here
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