A few days ago i had this problem that was a headache.
I was doing a multiplayer map.. when i wanted to test it, just wasn't there in the list... At some point in the mapping process, changed Map type to singleplayer in the "Scenario Properties" menu (if i changed it or if it was a bug i don't know). The thing is, changed it back to Multiplayer and saved the map and then exported the package. But the map was still missing in custom game map list. Every folder was Ok, -dev command didn't help on finding it, various threads in steam forum and in here with same problem but no solution.
The thing was, every time i changed singleplayer label to multiplayer, saved it and reloaded it, singleplayer was still chosen. So I made a new map and to see if worked with just the minimal requirements, and it worked. Then opened both "MapName.options" files with notepad++ and changed everything that looked different to the simple map that was working (aside from mapsize). And it worked. I don't know if i messed up with something while changing the code via notepad++, but it seems that everything is OK (played the map a few times).
If you have this same problem. Hope it helps you
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p.d.: My english sucks