If you have just set up contracts and each corp is set to contract over 50% of it production the corp will retain the contracted amount from the first months supply(to cover any possible shortages) and then deliver it's orders on the second month.
So if you set up a supply order for all the monthly produce, it will take a month or so before the corp has enough to send them out.
You can change the method somewhere in game from retain to don't retain.
I'm sure thats how it is anyways