I've been helping a friend out to add some improvements to his house, and we've run into some issues with a 3-way switch/light setup and could use some assistance. For context, he bought the house ~3 years ago, and it seems to have been a rushed flip that was sold to him, which has caused him several issues.
The kitchen/dining room/living room area has two wall plates with 3 switches each, which are 3-way switches that control the dining room overhead light, the living room overhead light, and a set of 3 overhead LED lights in the kitchen respectively. About a year ago, we replaced the living room and dining room switches with smartswitches, and have had no issues since.
We recently tried to do the same for the set of 3 overhead kitchen lights (all 3 were controlled by 1 switch), however after installing the smart switch and turning the breaker back on, the overhead lights wouldn't light. We took photos before and after, and all installation was the same as we had done for previous smartswitches. Attempted to remove the smartswitch and reinstall the old switch, but the lights would still not light when flipping either of the 3-way switches that previously controlled those lights.
We had some spare overhead LEDs, and even tried replacing all 3 with new ones, but still no light. The odd thing here is that, using a voltage tester, I can confirm that both of the 3-way switches (now returned to original switches) when toggled cause the LED lights/wires to the LED lights to swap between registering a voltage and registering no voltage, so SOMETHING is changing when the switches are flipped.
Not sure what next steps to troubleshoot this would be, the only thing we haven't tried is replacing both switches, but given that both change the voltage detection of the lights it doesn't seem to be the issue here. I have VERY thorough before/after photos and can confirm that everything is currently installed exactly as it was before when things were working.
What would be some suggestions as to next steps here? Should I remove both switches and re-install with the typical hot->2xtravelers first box then common->2xtravelers other box config? Is there any other testing I can do to determine the cause? Appreciate any help, and if the answer is just "time to get an electrician" that's fine too!
Edit: Before images for reference on the switch configuration: