Being a retired controls engineer who's last job was working with Extreamly fast food ovens, I know a bit or three about wring, PID's, thermostatic controls at the like.
The power supply is suffecent, the MOSFET is fully on, the wiring has been triple checks, and voltage drops on all the connections have been made. At the heater, I have on the heater itself (not the wires going into it) the full 12.3V on it, so it's got all the volts, thus pulling all the amps, therefor making all the watts it can.
The bed is only 1.4 ohm, so it's only a 100W heater at 12V. The heat rise and such it's operating exactly like a 100W square foot heater. My SeeMeCNC Rostock MAX2 has around heater, that's a little smaller, at at 300W it barely gets to 100C.
Either my heater is defective, or this heater could never get to temp. I suspect that this heater is actually designed for 24V (therfor a 400W heater). I'm temped into hooking up a center tap and rewire to a 0.7 ohm, 200W, execpt I doubt that its enough.
The correct solution is like what 713Maker suggests, with a 120V silicone pad heater, and a SSR. If you do something like that, remember that the classic brick SSR's are designed to be mounted to metal for a bit of cooling when dealing with high currents. 750W ( 6 1/4 amps) is on the edge.. you might get away without cooling... but you might not. Hopefully 713maker will also make the electronics out of Aluminum also, and put a couple of tapped screw holes for an SSR.
PS: Last oven I worked on, you might notice at Subway or Starbucks. I designed the control board and wrote all the firmware for it. This is NOT A Microwave, it's a really fast oven that really cooks Pizza in 1min 10sec, Toast a sub in ~22 sec).
The secret is "air impingement"