LV8729 motor current setting

I am getting strange results setting up a motor driver for my printer head changing mechanism. I am using a LV8729 and am driving a NEMA11 which has an Imax 0f 0.67A. I understand from the data sheet and other sources that IOUT = ( Current setting reference voltage / 5 ) / 0.22Ω. For 0.5A the setting should be 0.55V but this won't even move the motors. Increasing the trimmer voltage to 0.91V the motor draws 0.195A and the motor does move but is easily stalled - also, it does not get more than slightly warm to the touch at about 45°C. Cranking up the voltage at the trimmer to 1.1V the current drawn from the 24V supply is now 0.3A and the motor does get fairly warm, though still not hot. The motor is quite noisy at creep speeds but very quiet above about 150RPM.

Any thoughts?

Mike