Lets just get the right angle on this ( couldn't resist that)
From what you are saying you have your step per degree set correctly and your velocity set at 2000 degrees per minute - so, if you do a G0 on your A axis - does the thing turn at 5.5 revs per minute - or 11 seconds per rev (approx)
If it does then Mach is working correctly and the problem obviously lies in your program.
When using two axis (or more) at once, Mach moves the slowest axis at its fastest speed, all other axis are slowed to start and stop at the same time. I don't know whether this could affect it.
I see you have said that your program is limiting the axis to 6 - the program says inches per minutes - and the machine is using the 6 and converting it to degrees per minute. You might be better to alter the A axis to linear motion, and alter your steps per to the correct number to move the cylinder round one inch ( and this will clearly depend on the diameter of the cylinder). If it is something you do a lot of, then it is worth caculating it.
If it is a one off job, then keep on rotary but use your speed in inches multiplied by 33.3/D ( to convert to speed by degrees) to get the correct speed. Check the maths, I think it is right, I am a bit off song tonight.