The way that the Lore missions have been organized a new character can level to RA LH3 by the end of the Romulan mission series. Unfortunately, the player would not be able to access the Borg sector missions until he/she completes all of the Cardassian sector missions. This means that a player can receive a prompt that Borg Sector missions are available to him/her upon promotion to RA LH3 but the player will NOT be able to access them until completing the Cardassian missions.
Basically, the game progression needs to be tuned better with skill points adjusted accordingly. I believe that the problem probably occurred when the Reman weekly series was integrated into the Romulan story line.