Taim made a big point of keeping the gates open to Elayne so that food could keep coming in. Nobody except the Ashaman themselves knows how many people are in their city, they could easily be bringing in a lot more than they need and then gatewaying it north, without rousing suspicion. The dreamspike currently there wouldn't interfere because iirc it can be set to allow certain people through.

The DF channelers supplying the trollocs could also be plundering food (or people to be used as food) from somewhere offscreen, like Shara or Seanchan. Both those places are in chaos and would be mostly unable to defend themselves, and they're nice and distant from Rand's corner of the world so they could pillage all they wanted without raising his suspicions.