What owltoid says is correct. You have to be within a minimum levels between the 2 accts to gain raf Exp. This is why alot of multiboxers go the route of A->B->C->D for their RAF. So that A can boost with a high level toon. while B C and D still keep the RAF bonuses.
If you are planning on boosting yourself. the easiest way to do so would be to get another acct. Since you have A->C and B-D> currently.. You can link another acct to C so A->C->E this way you can boost with A. If you can find someone else to boost you you don't need the 5th acct.
Otherwise just like Owltoid said. Restart a new toon on A to level with the rest of the new toons and level them together.