With drafties, IME, it's easier to work on their vertical flexion later on, after they've got good, solid impulsion from very light leg. Because many of them are "lazy", the impulsion is the hardest part.
When I was dealing with starting Rafe, I had to work really hard to hypersensitize him to all my cues, which often could have been construed as being mean to him if seen by an outside observer. I would get after him hard whenever he was sluggish to anything I asked of him and I skipped over much of the force progression. I would take it right to about 8 if he didn't respond to 1.
I would give him very light leg and if he didn't respond, I'd take in with the over-under. If I picked up a rein a little and he had that feeling like he was grudgingly obeying, I'd bump him firmly in the mouth, give him the heel and/or the over-under until he was almost jumping whenever I'd pick up the rein.
Then, when he was almost over-reacting whenever I'd ask for anything, I started to ease off and let him mellow out. Now, he's nice and soft and responsive to light cues from both rein and leg.
In his turns, he's still pretty slow but I'm accepting of that because he's turning correctly and keeping his pivot foot planted. I can start asking for more speed later if necessary.