Check the battery volts at the battery side post on the contactor without the lift cut on,(should be in the 27 volt area been a longtime) then turn the lift on and check the volts going across the 2 posts on the contactor with it pulled in. A worn contactor can cause volts drops as well as drops through the emergency disconnects and the key switch.

The volts go from the batteries through the contactor then the ED switches and end up at the controller all the lift knows is the voltage at the controller.

EDIT most of the time the contactor is the problem because of volts dropped but you can loose volts at each point before it gets to the controller.