I've no idea what fat head lavender is, never heard of it but to get normal lavender to succeed it needs poor soil and very, very well drained soil and to be in full sun.
Trim it back quite hard after flowering and it should be fine.
You shouldn't feed it because it thrives in poor nutrient soil.

Lavandula 'Fathead' its French Lavender Loiuse. I didnt have much luck with it either till I realised it likes pour/dry soil, I would just leave it alone and wait! As I found the small plants were quite slow to take off. if its in a pot might you be under/over watering it?

Thank you both. Thats what i was wondering Drc never know how much water they need. I'll just go with the flow [pardon the pun ! ] and let it dry out a little see if that helps.

Might need a bit of protection in winter, not as hardy as English lavender.

Lavenders really don't like being damp and are very good for dry conditions in the garden - I add some grit and sand to my lavender soil because as DRC says it doesn't like ordinary soil. My fathead that I bought a few weeks ago is in a pot and doing very well and I try not to water it too much.

Good Luck with yours.

It's in a multipurpose compost will it be ok ?

You'll have to make that compost much looser, these plants like very thin poor soil so add some grit to it to emulate its natural growing habitat.

I wouldnt use compost at all just fill the pot with your poorest/gritty soil, water in and then only water when dry.

Thank you all for your replies,i shall add some grit and water sparingly and hope for the best !