(Re) MOSFET as LED current regulator
Maybe my input voltage is not enough?


It looks like it.



Table 1. From the datasheet.

With 2.9 V typical Vf and 3.5 V max you would require between 20.3 V and 24.5 V across the LEDs and another couple of volts across Q2 and R2.

Solution:


Increase the supply voltage.
Take out two LEDs.


Simple tests:


Measure the current.
Measure again while shorting out two of the LEDs.