Why did you put down plastic? That is preventing the soil to drain properly and the thyme is most likely water-logged.

I did that because it was an old unkept, very weedy area . the soil needed amending along with major weeding. I weeded, layered down newspaper and some thin plastic then put quite a bit of soil on top.. I'm guessing 6 or 7 inches. the paper and thin plastic was to try to control any new weed growth,

Weeds sprout when they are on the surface--so putting plastic 6 inches down has not helped at all. Dig up your plants, get the plastic out and replant. You'll have MUCH better success. Thyme is a lovely groundcover--I use it extensively and have 4 or 5 different types, but it is NOT weed suppressing and must be weeded. If you want a seed suppressing groundcover, look at either Geranium macrorrhizum for shade or G. 'Biokovo', or 'Karmina' for sun. Also weed suppressing are most groundcover sedums. (sunny areas)