Which specific assignment are you referring to? Anaprolean Fever, Correllium Fever, etc - which variant? Or do you mean to say that none of them are coming up?
They aren't disabled, I can tell you that much. But it will take more investigation to see if there may something wrong with their availability.
None of them have ever showed up. Sector Assignments, Shipboard Assignments, your Officer assignments (First Officer, Tactical, Security, so on)... the second mission never shows up, with any of it's 5 variations.
It doesn't matter if you're lvl 22 (when you first get Duty Officers on the KDF side), or lvl 50; it never shows up regardless.
As has been said by the other respondants, its all 5 versions of the quest. I have been to all the suggested mission spawn loactions many times, and none of the second part of the five assignment chains pop up.