request Help to troubleshoot GaN mosfet circuit error

[jf_NH]

Hi
THis is my first post as I found this forum Yesterday through eevBLog youtube channel
My question is rather simple but need some context:

I want to learn GaN mosfet specifics and for that embarked in  the following project.(in insight maybe a bit crazy...but I like challenge)

Starting from this:
https://www.digikey.com/product-detail/en/transphorm/TDINV3000W050-KIT/TDINV3000W050-KIT-ND/8322117
Transphorm has published the eagle files for this project so I started from them and I simplified it in the following ways:

1-I kept the power and driver sections unchanged

2 Instead of the inverter logic I just put 4 pwm input that I  power with 4 channels function generator.

3 I removed all the filtering in the main circuit So Ideally I should have an alternating square wave as output  ( and I want to see all the non ideal specially at higher frequencies)
 4 I removed all the sensor required by the original microcontroller logic
5 instead of bootstap circuits On the high side as in Transphorm's  I used 4 insulated 12v power supply one for each mosfet.
6 I added a 0.1 Ohm current sensing resistor between lowside mosfet source and power ground
Ultimately I want to go to high frequency switching where my interest lies)( reason for the insulated power supply on the low side mosfet

I build the pcb and started testing with either no main DC voltage or relatively low voltage DC

The problem I am facing now is the following
First I want to test each mosfet separately
For that, I connect a 10 Ohm resistance between the source of the mosfet to be tested and the power ground
My dc source is connected to the drain of the mosfet to be tested
1 Without the main DC powered I sent a pwm signal at very low frequency ( 0.1 Hz) so I have time to see things and perform manual measurement as needed,

I track the gate to source voltage on a scope and it rises as expected to 12V.

With Main DC OFF, I measure the Drain to source resistance with a multimeter and I go from "Infinite out of range" to 0.1 Ohm for 0.05 Ohm theoretical as per the data sheet.

So it seems it works??...no so fast...

My problem is that when I power the main dc to any voltage ( between 3V to 20V so far)  the gate voltage measured with respect to source is still a beautiful flat 12V
...unchanged compared to the waveform without DC
but NO current flow in my resistor so it seems the mosfet is not closing....

and yes I checked all the connection and continuity :

 the main test circuit is just a power supply a resistance and a switch.
So apparently
The Mosfet close when no voltage is applied to its drain and fail to close when some voltage is applied to it

I repeat this procedure for both high side and low side mosfet (all 4) .

So I obviously made a design mistake somewhere in "trying" to copy and simplify the transphorm design but I cannot see it.

My question is what kind of testing could be recommended to find this error??

I can publish my design if someone want to have a look at it but I want to learn and am not asking for someone to fix my design for me.

...and again it is just ( or should just be..) a straight "simplified" copy of Transphorm's

Any advice will be welcome..
Thanks in advance
JF