You may need to tear it down and find out what the problem is. That is a fairly easy job.. I've done it a few times.
You should obtain either a rebulid gasket kit or some universal gasket material (The latter at auto parts stores.... Generally not auto departments but true auto stores) as you will likely need it.
I use mine only occasionally and the dang motor siezes up between uses.. I now (finally) know how to fix it without a tear down, and may well know how to prevent it (But won't know if that works for a while yet). more if it works.