Find the control where the tooltip shows on as you move the mouse above, we will name it "<control>" here.
Either search in the lua files for OnMouseEnter or at the XML files for that.
This control could be a button and a texture control, which should be behind the button, shows above it now -> making the texture intercept the OnMouseEnter of the button and thus it looks as if nothing happens anymore as you move the mouse above that control.
To fix this, and you are right, you need to set the draw tier/layer/level of the button to something higher than the texture, which should show behind the button.
You can do this by changing
Lua Code:
<control>:SetDrawTier(DT_MEDIUM)
<control>:SetDrawLayer(DL_CONTROLS)
<control>:SetDrawLevel(1)
Or if the control is defined in XML only you must add the attributes at the XML line where that control is defined, e.g.
Code:
<BUTTON name="<control>" ... tier="MEDIUM" layer="CONTROLS" level="1" ...