Yeah, I've also noticed objects not spawning into a game if the connection isn't too great - even with only two players! My educated guess is that the object spawning happens around the time the Host enters the game. If a Non-Host player doesn't enter into the game quick enough, he'll miss out on some object spawns that already loaded for the Host. I hope it's fixable....
The reason I think it works this way is because this is what happens with Scripting Timers. If you setup a Timer: On Once at the beginning of the game, with a 0 second delay, then there is a very good chance that the timer won't fire for anyone who isn't the Host. If you actually give it a delay though, say 5 seconds, then everything will work perfectly fine for everybody (because by 5 seconds into the game, everyone has loaded into that game). Note that all this is still before the "Deploying" 10 second countdown has finished, and so is before anyone has actually spawned onto the map.
I suppose that, bearing that in mind, the issue of certain objects not spawning could maybe be resolved by having all the objects not spawn into the map until 5 seconds in? A bit time-consuming and tedious but it might act as a temporary workaround to the problem...