Plants usually don't bloom because they don't have enough phosphorous
in them. If you will look on the back of a plant food container, there
are three numbers: 15-30-15. These three numbers represent
Nitrogen-Phosphorous-Potash and each one is representative of what a
plant needs. If you want more leaves, get a plant food with a higher
first number, such as 30-15-15. If you want more flowers, get a plant
food with a higher middle number, such as 15-30-15. If you want more
fruit, get a plant food with a higher last number, such as 15-15-30.
The most common (and overused) type of food you will need for your
lavender is Miracle Grow, which is 15-30-15. Once you apply it, watch
out--that thing will never stop blooming. But don't use it on your
foliage or it will force them to bloom too.