Sometimes when you log on after the tri-hourly mission reset mark, it looks like your game bugged out and didn’t actually reset your missions (so you have to wait another 3 hours). BUT I have found that if you simply restart the game it usually fixes itself and gives you those missions again.
I also think that joining a game and then leaving also has this effect.
None of those “fixes” work, like ever. It is a server thing, so restarting the client will not fix a thing. I’ve had it happen to me before but ever since the Phoenix update it happens to me almost daily.
I don’t really need the extra credits from missions at this point, but for someone new to the game who wants to save up some credits for a new merc this is a big issue.