Laser power is very low in TTL mode

[sp2567]

Hi I bought a chinese (like many people) 3,5W laser and I have a problem.
I use an arduino V3 shield and the laser has it's own power supply

The problem is that when the laser isn't connected to the shield (in a few words not using TTL) works fine. But when I connect it to the V3 shield the power is very low
The command I use to turn it on is: M03 s255
Any idea?
Thanks

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