what map is that on?

This is something like what I am suspecting to be the problem. THe more experienced map people should be able to help. Would the banner set up as its own file? Aww saw this on the same server no one else could see it on so that indicates something different about his client than anybody elses. The rest of the map ran fine for everybody else but this one particular item does not show for anybody but Aww who created the particular map.

if you made a custom decal you have to make a  .vmt file for the decal. you have to do this otherwise the rederer can't properly interpret your decal.

 This .vmt would look like:

"LightmappedGeneric"
{
    "$basetexture" "decals/mydecal"
    "$decal" 1
    "$decalscale" 0.10
    "$translucent" 1
}

It's important the "$decal" parameter is included. Without this parameter, the material will not clip or be projected onto the world properly.  hope this helps.

then embed the .vmt file into the bsp with pakrat! that way when people download the bsp, they also get your snazzy decal.

and this is the reason i don't mess with custom decals, its too easy for things to go wrong.  (that and i'm lazy)

here is a trick i figured out a while back.  take whatever you want as a banner and make it your in game spray.  then once you have loaded up the game with that spray then quit the game.  then go into hammer and type something like "rebel" in the texture box, then nothing should come up.  now delete the word from the filter line and scrol down.  you should see your spray as a selection in the textures.  you can add it this way (as a texture or as a decal).  the only problem with this method is you can't make it very big, but it works.

yeah it can get complicated and you now know why i don't bother with it.  if i feel real keen, i might add some small ones in the spawn rooms, but usually i don't bother t all because its more trouble then its worth.  its can't really personalize the map because if somebody wants that map on their server, then all they have to do is decompile the map, select the decal, and click delete.  then re-compile the map and there goes the custom banners.