The motor on your A axis would seem to be the trouble.. When I was setting up my system, I wired my eight wire motors in series (I didn't know any different) and I could only get 4 inches per minute out of them before they missed steps (they didn't just miss steps, they stopped althogether).
I rewired them in parrallel and the difference was astounding - straight up to 40 inches per minute. I wound that right back for reliability but I can still get 20 inches per min out of them - reliably. Not up to your speed, perhaps, but this is a steel lathe I am talking about.
If your A motor is eight wire, in series, try it in parrallel, your Gecko will take it. My drivers were 2.5amp until I went onto Gecko, but even on that, performance was must better in parrallel than series.
My motors were rated at 2.5amps per coil (at 7.5v). With the Geckos I have increased the current set to 4 amps,(feeding two coils in parrallel) and just lately put my voltage up from 24 to 36 volts. The motors will get hot (remember they are on all the time, even when not moving) - but if you can touch them, that's fine, if you can put a hand on and keep it there they are running cool. Reliability has improved tremendously.