MOSFET doesn't put out desired voltage
Are you by any chance trying to use it as a high-side switch? 

I believe so - in which case you're actually using it as a source-follower instead of a switch. This is confirmed by your measurements, where Vs = Vg-1.5V approx. It's not switching, but operating in linear mode and dissipating a lot of power, supplying a fairly accurate Vg-1.5V to the load.

You have 3 options:
(1) generate a gate voltage ABOVE the supply (18 to 20V) to switch ON, and 0V to switch off.
(2) use it on the low side of the load with 12V gate voltage. This is the simplest - a conventional low side switch - and offers the best performance.
(3) replace it with a PMOS FET as a high side switch. Then 12V turns it off and 0V turns it on. But it might be difficult to find a PMOS with such a low ON resistance.