Are my bees being robbed out?
With the info you have provided, It's hard to say.
Perhaps give us some more details of their spring & summer activity.
I'm really suprised the "oldest" of your hives has no stores,
this leans me towards the strong possibility they were robbed out.
Since you have five at the remote yard, why not bring a couple
home and see if they start putting up stores?
At least then you'll know if the location is to blame.
As to the question of robbing, maybe put up a robber screen on
just one of the hives...