I had ajuga doing well under a pine tree – for a few years – but then it all died away.
Mind you, the pine is of course growing, now fairly tall. Could there be something about the pine?
Hello: Ajuga have lovely coloured leaves and an outstandingly blue flower, so no wonder many gardeners choose this plant for a shady ground cover. But ajuga reptans thrives best in a fertile moist environment- the type of environment that is seldom found under a tree. Partial shade, not full shade seems to make them the happiest. Your plants likely thrived for a year of two because of the moisture retentive nature of the soil in which they were potted, but over these years the dryness and increasing lack of sunlight and nutrients as the pine grew caused them to fail. However I would try again, perhaps orienting them to where they would receive some sun. Water them thoroughly at least once per week (more often in the first few weeks). Depending on how large the pine, don’t count on rain to supply the necessary moisture. If the pine boughs drag on the ground you may consider removing the bottom few branches (a good hair cut).
Add 3 or 5 cm of a manure/ peat moss/compost blend to the ground under the tree early each spring. Do not add a deep layer of soil because it may harm the tree (see first link below). Please choose healthy but small ajuga plants to avoid digging larger holes than necessary to prevent damage to the evergreen’s roots.
If you don’t want to try ajuga again the second link makes some evergreen suggestions. I might also suggest Lamium maculatum, wild ginger Asarum canadense or asarum europaeum, or sweet woodruff Asperula adroit. Another alternative is to cover the area with a shallow layer of bark or small stones.