I haven't seem to have any problems getting them up the ladders, other than they usually don't go up ladders well, they get stuck at the bottom a lot for some reason... so then you gotta try and make em jump onto it, but make sure they line up, and still hit the waypoints right and stuff...
just make a new set of waypoints for each ladder you find they get stuck on, and keep workin' it till they can get up and down 'em... more often than not, DON'T make them climb down a ladder unless its absolutely necessary, cause one bot will go down, and one bot will go up, and they'll stay there till one dies... and even better, if you can keep the up and down pathways out of alignment with each other, it helps a lot too.