MOSFET Off When Load Voltage Applied
4 main choices:


MOSFET is dead or misconnected
12V Power supply cannot supply about 12W needed.
Buck converter cannot supply 1.5A / 9W needed.
Load is not really 4 Ohms.


FET datasheet here

MOSFET is dead or misconnected

Pinout is standard as all getout as per page 2 of data sheet.
On flat surface, metal down,  tab away from you legs' towards you connections are gds left to right. 

In circuit, power on, short ds with wire.
If result is as when using FET fault is in buck or psu or FET drive.

Apply FET drive.
Measure voltage gs AT FET.
Should be 5V. 

12V Power supply cannot supply about 12W needed.

Load 12V with 12 Ohms or less resistor.
Ensure 12V does not droop.
If supply has variable current limit it may be set to below what is needed. 

Buck converter cannot supply 1.5A / 9W needed.

Supply 12V to buck converter.
Apply 4 Ohm load AT buck converter outputs across Vout-gnd.
Should supply load.

Load is not really 4 Ohms.

Apply 6V to load.
Measure current in load.
Measure voltage across load to check.
Apply Ohms law.

If all the above fails then you have a misconnection or are using insulating tubing in place of wire.

Remedy any inconsistencies found above.

Your circuit now works.
Q.E.D.