If you need help please enlighten us with more info ...like what copy of maya you use 4.5,5,6,6.5,7, and of course which plugin is in question...
both wood help in aiding you.

I don't know what version of the X file exporter you're using for Maya but I know in a previous version that I actually had to paint the vertex colors to get my textures to show up in the game editor that I was using. Currently, I'm just an artist and have limited programming knowledge, but the programmer I was working with at the time said that painting the vertex colors fixed the texture issue that we were expereiencing. If you are having the same problem that I was experiencing, which is textures didn't show up at all (transparent geometry) or the mesh is showing up black then this will hopefully fix it.

If you don't know how to paint vertex colors here's some quick instructions. With the textured object selected that you wish to export go to the Edit Polygons menu and mouse down to colors-> which should be near the bottom of the list. Go to the Paint Vertex Color Tool options box. Change the color to white and click the Flood button. The object may brighten slightly, which is a result of the brighter vertex colors overriding Maya's hardware rendering and shading. In the game editor though, this should allow the texture to be displayed. Another issue that we ran into was the normals for exported meshes. For some reason we ended up having to reverse the face normals for any mesh that we wished to export. We eventually resolved this issue on the programming side, but if you're still having issues you can also try this technique.

Hope this helps.
-Matt