That does happen when you have the coolant running and the valve closed, but you DO need to do that at times.
Make sure all of the surfaces are totally flat, super CLEAN, and deburred. Use a real good grade of silicone sealant or the equivalent. That stuff will seal anything if used right.
Make sure there are no burrs on the bolt holes on the plate either and bolt it down securely. Now comes the hard part, follow the directions on the sealer and don't use for at least 24 hours probably. After that, do not close off the coolant valve for a few days.
If you do that, it will last a long time. If you hurry it, it will fail again soon. Gaskets are fine, but do it the same, let it wait!
Mike
Two Haas VF-2's, Haas HA5C, Haas HRT-9, Hardinge CHNC 1, Bother HS-300 Wire EDM, BobCAD V23, BobCAD V28