Hi maite,
A friend of my went to Paris during Easter break and she intended to help me to buy/order a Birkin. At last, she could not do both because (i) there was no B when she visited the store, and (ii) both FSH and George V rejected to take order at that time. The SAs told my friend to try again in September for ordering.
Having said that, I saw from time to time people here can scrap a Birkin in FSH. So I guess if you are not eyeing at a particular colour/leather/hardware combo, it is easier to buy one.