I've found that USS often will only spawn what you need for the mission in your mission list that you took on first.
So say you got 3 specific missions that is not a simple trade mission, for example - and in order mission was taken:
A: Find and return a Black Box
B: Kill Pirate Harr
C: Kill Traitor General Trenchfoot
Until you've completed mission A, a USS for B and C will not spawn. No need to return the Black Box in A, but you must have collected it. Similarly, a USS for mission C, will not spawn until you've killed the Pirate in mission B.
Speaking of the Rank-Up missions - often you get several of the same type. Say you get the a mission to Kill General Trenchfoot, you've not completed the mission and dock at a station, you then get a mission to kill General Pacifist. Doing either one of these will earn you that promotion, but then doing the other won't.