Motorshield V1 power is low

I have this Version 1 motor shield hooked to a Nano.
Everything works fine BUT.......
I can't get more than 11.0Volts to the motor, maximum. (speed=255).
The motor should only draw about 500 mA but to be on the safe side I've doubled up on the L293 so that shouldn't be an issue anyway.
My input is 12.4V DC. I'm using M1 but have tried M2 (no change)
I've tried to look up some info about all this and can't come up with anything to explain that large voltage drop.

Any help will be appreciated.


1.4volt dropout is very good for an L293D.
The datasheet states a typical dropout voltage of 2.6volt@600mA in H-bridge mode.

Adafruit's v2.3 motorshield uses TB6612 mosfet drivers.

Oops. Don't know how I missed that. Sorry.