Each motor is on a 30 amp breaker subpanel both fed into a 60 amp breaker in the main panel.

Voltage drop is can be the result of any one or more of several factors.

Distance of wiring. Either/or utility wiring, feeder wiring, subfeeder wiring.
Size of wire.
Loose connections.
Low voltage from utility.

Motors draw 6-10 times the running current to start. Long wire and/or small wire, while possibly sized to handle the running current, may be too small or long to handle the high starting current of motors.

If you get back with more details, perhaps we can work this out.