HAT 5 volt issue

[ceeller]

Hello all. I've completed assembly of our PlanktoScope but have noticed that we have an issue with the power delivery across the HAT. When plugged into the 12v power supply, the Raspberry Pi does not power on. I checked the test pads on the lower right area of the HAT and get 12v from the GND to +12V pads. When I test from the GND to +5V pads I only get 0.25V. (image below of the pads)

Can someone advise where I should start with troubleshooting? I have basic metering and soldering skills, but not much experience in circuit troubleshooting.

Thank you all!