Can the companies that need to buy afford to buy the plastics?

well as i understand it, the fleet goes to a different station each day and tries to sell its goods. so it may be a while til it reaches one that can. for now i just overide it by clicking on 'trade' for the ship or ships and telling it a specific station to sell at. then go to the trade route and skip to the next step.

If overriding works, then something is weird. Did it work before?

If not, if say 'load' and 'unload' you need to say 'buy' and 'sell'.

it works some of the time. as far as i can tell its when the fleet is not currently trading. the only way i know is that the 'trade' button for the ships is not greyed out.

You can check the standings and maybe the corps in that section can't afford to buy anything.