Lavender wants full sun (8+ hours daily), a lean and very fast draining soil, no fertilizing and minimal watering once established.

I'd wonder about your soil. Regular potting soil is often too rich and too moisture retentive for lavender. Something like a mineral based cactus mix is more desirable. btw, rocks at the bottom of a container have the opposite effect on drainage - they actually slow water movement, so these should be skipped.

Watering once a week may be more than necessary, but that all depends on the potting soil used and how much moisture it holds. And get rid of the bark mulch - containers do not require mulching and that could also be holding more moisture in the soil than is advised.

As well as all the above, the container does have drainage holes in the bottom, doesn't it?