Reading other forums it sounds like sometimes that happens. What you apparently need to do is try to make enough fusion matter to go to the next level, then more missions will open up. Do all the lower level missions you can, farm monsters, and do speed runs. There are a couple nanos who have powers that increase the amount of FM you get at the end of each mission, but I think the only one you'd have gotten so far is Buttercup (level 1)'s Scavenge skill. If you equip that nano before completing a mission you get like 20% more FM, which can be a nice bonus. Since the tutorial directs you to choose the stun power you'd have to switch to it, which isn't cheap.
But that's up to you.