I don't know where you live, but lavender needs a very well drained soil. You might want to try planting them in a raised bed and see what happens.

Here is some more information on lavender:

French Lavender Lavandula dentata Tender perennial. Overwinters in Zones 8-10; should be potted and brought indoors in colder climates. An herb garden standard, French lavender produces wave after wave of lavender-purple flowers in midsummer. The foliage is gray green with a serrated edge. Its perennial in zones 8 through 10. Northern gardeners should grow it in pots and move indoors during the winter. Or, take cuttings from garden plants in the fall.

Planting & Care. Plant from containers in well-drained soil in a sunny, sheltered location. Expect only a few blossoms to appear on one-year-old plants; maximum flower production typically develops by the third year.

Cut back flower stems after bloom period has passed.

Rejuvenate old plants that have become ragged or woody by cutting back branches.

Don't shear but prune branches selectively to retain the natural form.