My headsail unwinds completely but I can only pull it in a couple of revolutions.
I have unwound it completely and have still the same result no matter how many revolution on the furler.
What am I missing?

hi, i would look at your furling halyard. after replacing mine it was perfect and so smooth and effortless to furl.
maybe you could try soaking the halyard in nappy san or some fabric conditioner for a few days then end for end halyard when you refit.
is your halyard too big a diameter and filling your drum?
good luck, it should be an easy fix

Check for halyard wrap. This explains it best

If the top swivel is too low, causing halyard wrap, easiest solution is to add a pendant.

Same thing happened to me. One of the problems was a split bearing seal resulting in a bearing
going rusty, but, the main problem was the top swivel being too low. This resulted in the halyard
wrapping itself around the forestay as I furled. I had to stop/start the furling action to furl the sail.
I took it to a rigger who adjusted the whole thing properly and made new strops, and it now works
perfectly.

Couple of plastic balls on the halyard will just rub against the foil and stop the halyard wrap problem too if the angle is not enough.