the quick and dirty version.
A axis is angular, as said above.
microstepping no more then 10
Zero A axis
use MDI to rotate 360 degrees (G0 a360)
if it goes past 360 degrees then your steps per in motor tuning is to high.
if it rotates less then 360 degrees then steps per is to low.
Change your steps per for the A axis, click the "SAVE AXIS SETTINGS BUTTON"
back in mach, zero the A axis work cords. they will change.
repeat steps.
I will also mention that i have had an A axis velocity set really high once. The axis would run smooth at 3000 mm a min. All other axis were set to 600mm a min. When running a 3d toolpath I would get missed/lost steps. Tracked it down to be that the difference in the A axis velocity. So tune the A axis close to the other 3