Troubleshooting Main Driver Subassembly

I have taken over a set of FlowIO components from a previous student and am currently trying to get it to run. However, I suspect that the main board must have some sort of open circuit or faulty on/off button. Hereafter is what has been done so far and the steps I have taken for troubleshooting. The components that I have available are shown in this image (+ the tools I use for troubleshooting):

The problem basically occurs when following the 'Main Driver Subassembly' page; after connecting a 3.7V power supply in step 9 (instead of the LiPo battery), when pressing the power button in step 10 nothing happens:

No current seems to be drawn, and the microcontroller doesn't seem to be powered as there's no LEDs lighting up on it. Also, when trying to read the voltage directly on the main board's 3.3V and GND pin (I have tried it both from the header pins as well the solders at the bottom of the board) I read 0.0V, whereas I would expect to have 3.3V there and this is presumably the reason why the microcontroller doesn't get powered:

To exclude the possibility that the Feather board is faulty, I have tried hooking it up directly with the power supply's 3.7V as described in the instructions, and everything seems fine:

Similarly, when connecting to the Feather board via Arduino on my laptop everything works as expected and I am for example able to upload the 'Hardware Test' Script. So the Feather board should be fine:

This makes me conclude that the fault must most likely lie with the main board, with either the on/off button not working properly or maybe some faulty open circuit. I would therefore opt for re-ordering/re-fabricating such a main board. Nevertheless, I am in no way an advanced electrical engineer and might have missed something in my analysis.

Finally, here are some close-ups of the main board the way I have received it after use of the previous student:

Any help / follow up would be highly appreciated!

Thank you,

Ian