All lavenders need full sun, dry well draining preferably somewhat alkaline soil poor in organic material and require pruning part of the green stems but not into the wood. If your conditions are not as described then the lavenders will be short lived. Most often than not fungi will attack them.

And furthermore your species, Lavendula stoechas, is the one which needs the most heat and the best drainage out of all the usual garden lavenders.

Ah interesting. Perhaps I should switch to another type of lavender then. I have heavy clay soil and rainy climate.

Or switch to different plants.. In any case avoid mulching lavenders. Maybe you can plant them in a well draining raised bed. English lavenders are more accommodating. Full sun is a must for them to bloom well.