Hi Amsterdam, lavender tends to start producing flowers in May and you should see lots of flowers in June and July, so it shouldn't be long now. Just out of interest, how old is your plant? Did you prune it last year?
You're doing the right thing about feed - they don't need too much of that. Don't over water either.

I would pick off the dead foliage but as it is growing in a container, when you water, you wash away virtually all nutriments. I suggest you give it a light feed with a general fertiliser, perhaps Q4.

They have been in there for a year or so and I did trim it back Last autumn. I might give it a light feed to wake it up! If it flowers in June/July it’d better get a move on!

Make sure it's getting lots of sun, too. Even here in the desert, they aren't really happy in the shade.

GQT today on Radio 4 said best trimmed in August.Means cutting flowers off but they suggested using them for sachets, pot pourri etc.

I go to Hitchin Lavender Farm once a year for lavender flowers (you pick your own) and make lavender pillows and soap but there's so many other things you could use them for.

I have given them a feed so now must wait and see.