If you look at the G-code, is it calling out any moves for the A axis? There should be commands like "G01 A 10.567 F900" If searching for A moves brings no results, then your program isn't going to activate the rotary table.
If the commands are there, but it's still not moving, check the settings in Mach3. The A axis should be angular and denominated in degrees, and there should be a number greater than 0 in the diameter field.