Could be bugs... over watered lavender might be susceptible to bugs. Otherwise, you might pull up one of the nearly dead ones and look at the roots, if they are black.. that's how they died, either over water, under water or something ate the roots, OR, sometimes compost makes a too fertile soil for lavender, depending on how much and type you added.
Those are just my thoughts.

My Goodwin Creek Grey died, too, branch by branch. It was in a pot - entirely possible it was over or under-watered, since children help me water and sometimes miss something, or drown it.

I had taken 6-8 cuttings and rooted them. The earliest cuttings flowered. I am interested to see how the little plants from cuttings do this winter. I was going to try to winter them over in the attic - cool but not frozen, in a southern-facing window, with sparse watering until spring. I have other lavender cultivars in the ground, all of them in their first year in a new garden. I'm looking for which does best in our particular corner of the world.

Heathen1 - what kind of bugs attackes lavender? We have white flies on the roses, earwigs eating the Rudibeckia, you get the idea. We got bugs. Is Goodwin Creek Grey just not a hearty variety? Suggestions for a more robust one? Thanks!

I planted lavender on the perimeter of my organic produce garden because I believe that many insects do not like the scent. I have a lot of marigolds in that garden, too, for the same reason. I only had trouble in the garden with one very destructive type of beetle. The beetles didn't go near the lavender.

I think overwatering might be the reason for the lavender dying branch by branch. It could be rotting roots. Lavender needs soil which drains well.

I can water mine quite a bit, but I also have sandy/gravelly soil. Haven't lost one yet in 5 years. I am thinking your clay soil is your culprit...drainage is just too poor.

Vera

I had to add gobs of sand and compost to our clay soil here in va., that and full sun,I haven't lost one, and they are huge after one season, I have lots of cuttings I will share with you if you want to start over,they grow fast from cuttings, vickie, tidewaterlilly

Where are you at? If you have earwigs, you must be in the west??? Sometimes if people know your conditions it can help diagnose. I live in Sacramento, I have heavy clay, yet my lavender thrives, but I don't water it much and we don't get much rain in the winter.
Instead of too much compost, I mulch, which helps the soil and doesn't fertilize too much.
Bugs? Maybe mealies, but I'd think they'd be pretty obvious. I moved and keep having an outbreak of spidermites that I didn't have 6 miles away.
But I'd think watering is probably the main culprit.