umm, why wont these units get on?

Im trying to get some units to board a unit that : load unload and cargo hold, but it wont work. I need to do it with triggers but ive tried to order: the units to "board" units to "load" the transport targeting the units to "board" and "load". why wont it work??(I know the trigger is fireing)

Do the units take up a space of "0"? If so, they aren't allowed to load onto zeppelins, or anything else, for that matter.

no, you dont understand. they can load, they load fine, but I need to do it with triggers yet it wont work