I have been stuck without any new missions for a long time...
I'm a commandar 4 (lvl 24), below are the missions I have available to me. I think they're all repeatables, please let me know if not.... (i haven't included the ones that are listed as dailys or that I know are repeatables)
Make sure you do all the Patrol missions available to you. These aren't repeatable and are a good source of experience. Make sure you've done the Breen and Devidian missions. Breen missions are in the Orellius sector. Devidian missions are in the Eta Eridani sector.
If you still haven't reached the next level, keep grinding exploration and/or defend the sector block missions.
For future reference you should do missions in this order:
Story mission for your level ONLY
Patrol mission for your level ONLY
Defend and/or Explore missions until you reach the next level
By doing it in that order you minimize the chances of reaching a point where you run out of missions. I'm not saying it won't happen, because there are some levels where missions don't exist for it.
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If you still haven't reached the next level, keep grinding exploration and/or defend the sector block missions.
For future reference you should do missions in this order:
Story mission for your level ONLY
Patrol mission for your level ONLY
Defend and/or Explore missions until you reach the next level
By doing it in that order you minimize the chances of reaching a point where you run out of missions. I'm not saying it won't happen, because there are some levels where missions don't exist for it.