Oh, you didn't say they were in pots.
Lithodora is a ground cover that grows by spreading. How does that work in your flower pots? Are you growing it just as a trailer, trailing down over the pots? Did it bloom at all this year? They only bloom in spring time.
I don't know enough to know from your description about your fertilizer. But you want to be looking for fertilizer for acid lovers:
look for fertilizer for azaleas, rhododendrons, etc. That will provide the acidic environment your lithodora needs.
Your soil may also be staying too damp for it. A cactus mix or amending your soil with part cactus mix might give it the well draining environment it needs.
It is a full sun plant. You said it is in full sun, but then you said it is on a balcony. A balcony is never full sun, since there is a building behind it. What direction does your balcony face and how many hours does it get direct sun?
Just because they were flowering well when you bought them last year, doesn't mean they really like your conditions in the long term. You have to provide what they want.
Did you repot this year? How big are your pots? (Pictures would still help). I had a jasmine that grew like crazy and never flowered. When I took it out of the pot, I discovered the pot was so full of roots there was hardly any soil!