After playing the progression:
Dm9 Gmadd6(add9)
FM7(13)
C9 Gmadd6
FM7(13)
Going back to the Dm9 right after this sounds pretty bad. I assume this is because the C9 implies a key change from minor to major (D minor to F major). If left out, this sounds fine.
Why does the C9 chord function like that? I think that the progression sounds nice by itself, but would like to repeat it once more but it's being prevented by that.
One possible explanation is that the FM7(13) is really the same chord as the Dm9; only the bass changes.
The FM7(13) will likely include something like E A C D up top (or something like it), which are the exact pitches you'll have in the Dm9. You may want to add in an F in the upper voice of the Dm9, but of course you had that F in the bass of the FM7(13).
Especially since you're playing this after what seems like a full measure of FM7(13), the Dm9 just isn't enough to actually sound like a change in harmony, and so the harmonic rhythm suddenly stagnates.
(Note: You could make a very good case that the C9 and Gmadd6 are the same chords with just a change in bass, but a) this could just be jumping to an inversion of the dominant C9, and b) this makes it a one-measure harmonic rhythm, not an FM7(13) that lasts for more than a full measure.)
One way that may add in some interest and allow you to move into a Dm9 would be to give it a little tonicizing chord. Something as simple as an A7/E that connects from the FM7(13) into the Dm9 makes the progression sound much better to my ears.
