I'm surprised you're having that much trouble. Teflon washers usually do the trick. If you do use silicone, use ultra black under the hexhead. Be carefull on the threads- I actually have had oilpumps lock up from stray chunks of silicone that got away. It doesn't take much to do it,trust me. Be sure to degrease the surfaces with brakeclean or something that will flash off and leave it dry.
IF none of that works, there's an old millwright's trick. Go to the hardware store and get some lead fusewire. Wrap it around the under the head of the plug and it'll crush forming a seal. It's softer than copper and usually does the trick.