My question is very general but I need advice on how to start the troubleshotting process

I have several low voltage light units on both sides of a brick path

They are on a timer unit which sits outside and plugged into a power outlet on my porche

2 of these lights do not come on [they are not next to each other

The bulbs seem OK

I can see part of the wires not completely covered by the dirt on the adjacent mulched bed and could do further digging if needed

? How do I start the investigation process

Thanks in advance