well drained compost and bottom heat?
It might be easier to do cuttings from an established plant, also in well drained compost.
I've always had the same problem! Though I 'aquired' a couple of cuttings from my local Wickes car park last summer and they have grown very nicely indeed on my kitchen windowsill, they even flowered at christmas, so they must have been fairly happy.
My Lavender seeds have refused to germinate, yet again. Year after year I try, and every time I fail. No idea what I'm doing wrong, like you, I follow the instructions to the letter. Very disheartening. I think I may give up on them altogether and just get some cuttings of these too...
Don't bother with growing Rosemary from seed or Lavender it ridiculously tiresome and a waste of money. Both are easy to grow from cuttings- you can take a cutting from a friend that has a well growing plant or take a cheeky cutting from a plant at the garden centre… I call them samples. I've tried many of ways they suggest for germinating lavender and rosemary, never once got rosemary to germinate and the Lavender seeds that I got to germinate made for the most dismal looking plants.