Damn so you basically just reloaded right before the mission after you found out which stock to invest inI invested with everybody. Everyone besides that vapid stock should be invested in before you start the missions. The Vapid one is the only stock that drops by half from I think 400 a share to 200. That one you buy up after the mission and it will skyrocket up within an in game day or two.
Edit: each one should be bought before it's respective mission. Just do a quick save before the mission to be sure. Don't be afraid to dump all the money into them.