Pump does not get enough current via MOSFET

I am trying to build the v4.3 circuit described here: Will this schematic work?

I am building just one branch out of the decoder IC to test on a breadboard. Component info is as follows:

1. The optocoupler is PC817 i.s.o TLP621;
2. The MOSFET is IRF630B;
3. The flyback diode is 6A4;
4. The resistor between MOSFET gate and ground is 5.2K;
5. The peak power rating of the pump is 2.7A @12V.

Now when power is on, RasPi is up and the input to the decoder is set correctly, the pump does not work.

I tried the following to troubleshoot,

1. Connected the pump directly to 12V input, it works. Connected a multimeter in series and found the current draw to be approx 3A to start and then dropping down to 1.8A;
2. Connected an LED via a suitable resistor across the output side of the optocoupler (no mosfet/pump) and it turns on and off depending upon logic input to the decoder;
3. Removed LED and measured the voltage across the output side of the opto-coupler to be ~12V;
4. Added the mosfet and measured the voltage across the drain and source with correct logic input to decoder. This measured ~11-12V;
5. Added pump (w/o flyback diode). Now with correct logic input to decoder, it does not work;
6. Connected a multimeter (in current reading mode) in series between the cathode of the pump and drain. It measures only 0.63A;
7. Replace the pump with LED and proper resistor and enable the decoder, the LED lights up.

It appears to me that connecting the pump via the MOSFET somehow impedes the current flow to a great extent even when it is biased according to specs.

I thought that the MOSFET in question is a voltage controlled device. So when a proper \$V_{gs}\$ is applied, it should simply conduct the current required by the load.

Why does it conduct only 0.63A when the starting current requirement is around 3A? Am I missing anything?