Neutral Passive units invisible?

Well, I just started map making today so excuse the obviously newbish question. When I put a neutral passive hero/unit on the map and then run the map the unit is nowhere to be seen...Is there something I need to do , like a trigger or something?

Thanks

i never herd of such a thing, but if it can't be fixed, it can be triggered, so just make a trigger that says
E: game time is 2 seconds
C:
A: change <unit> ownership to neutral passive, changing colors....
A: the same thing if there is more than one unit...

that's basic triggering, you can do it with one action but it will require a region....
but make sure that the units you put on the map doesn't have "shadowmeld" or "permanent invisibility"

Thank you , some of them did have perm invisiblity :)

There's also a melee trigger that removes neutral units from around the gold mines at which the players start at the beginning of the map.

Don't know if that relates to your problem, but it can definitely cause the symptoms you're experiencing, in certain situations.