Why are MOSFETs only giving 8.1V for LEDs, not 12V?
By my calculations you are sinking 6 amps:

72 W / 12 V = 6 A


I have some doubt that the Arduino board is designed to handle that amount of current through its ground plane. Try running a separate ground wire to the MOSFET sources.

I would also be concerned about the gauge of wire you are using, and whether the prototyping board can handle that amount of current.