Does this mean that if my source is 18V and my gate input is 5V, the MOSFET won't turn on? My understanding was that the advantage of a MOSFET is that you can control sources with large voltages using a small voltage input.
Just to update the OP's schematic with the accumulated wisdom gained from this thread:
Misunderstanding over grounds is such a common problem I wrote a tutorial about it.https://forum.arduino.cc/index.php?topic=653831.0
Your symptom sounds exactly like what would happen if you used a non-PWM capable output pin.