Explain please. We can't help you if you don't tell us what is wrong. Are you getting an error while your making your .mdx file? If you are, try restarting the Replacer, as sometimes it has these spazzing sessions, and a good restart helps.
The only way to edit a head and not be read by game is to edit a wrong head ot to have that model twice in override folder (you can do that in TLS). Put a screenshot from g-max and another from game, it will give us more info about the problem.
"If you must sleep on the same bed, at least try to dream different dreams".