So, we had this house built 3 years ago. We had lighting installed under the cabinets by the contractor. They worked great for a year-ish. Then they all kinda went out.

Went and bought new replacement (led) bulbs from the same place the original fixtures/bulbs came from.

Put them in and still nothing. I have a tool that tells me "hot" areas and these are hot according to it. I have the Klein Tools kit.
https://www.homedepot.com/p/Klein-Tools-Electrical-Test-Kit-69149/204367271

Attached are the pics. How can I have power, new bulbs and nothing. What am I missing?

Switch is on. Only one of 6 sets works and they are wired into the same switch.

Some of the under cabinet lighting will have a switch on the side, maybe turned off. Bulbs in correctly?

Thanks, switch is on and one set is on. The others are all dean and on the same switch.

The bulbs are in backwards.

Did you try reversing the bulb?

if they are led then reverse them.

if fluorescent make sure the tabs at each end are fully contacting the prongs.

Just use candles. Lightbulbs are overrated.

I think to switch to LED bulbs you need to change the ballast. I tried to do the same thing (switch to LED bulbs) and it didn’t work and I didn’t want to change the ballast, so I just went and got replacement fluorescent bulbs like I already had.

Normal LED bulbs need no ballast. They’re just regular bulbs wired to 110 vac on one end. The socket in the fixture only have power wired to one end, so the bulbs have to be oriented correctly.

There is still a conversion process of some sort. It isn’t as simple as buying LED bulbs and putting them in the fixture, is what I’m saying.

A lot of people wont believe this but Walmart sells some 4’ t-8 size LED lamps that will fit and work in both t-8 and t-12 fluorescent fixtures without having to change any wiring. I don’t know how they can make that work but they do.
In the OP’s case i bet the ones that don’t work just need to be flipped so the lamp pins are in opposite ends of the fixtures from how they are now.

If they're single-ended tombstones, they need to be installed in the correct direction or they won't work. Flip the bulb around as said above and see if that works.

These bulbs are 13w fluorescent. You can flip them any way you want, and nothing.

I put some of those little fixtures in my trailer at the lake, took them out a month later as all I had was problems with them.

turn them off, take an ice pick and very gently bend the contacts in the tombstones so they contact the prongs on the light.

Blame China