I have just finished a three-day slog performing missions in a three-system group to get the rank-up mission from Count to Earl.
Each rank-up has required more and more missions to have been carried out before the imperial navy rank-up (ascension) mission is offered. If you carry out as many charity missions (those which don't pay you credits for completing) as come up, that will help (not just the donation ones) as they count as "high" for your reputation, whereas fetch and carry are only "low". I didn't do many of the smuggling or assassination missions so I don't know if they are bugged but the others will get you there, you just have to be patient.
I suppose if you find that your have carried out a large number of missions for your allies without getting offered even a "serf" rank-up mission then maybe raising a support ticket will get them to check your status.