Unable to add person

Dear Sir / Madam

we booked a 4 sleeper cabin with only 3 people in the cabin, we are try to add a 4th person but unfortunately i am being told that the ship has reached it's capacity. Please explain to me how the ship has reached it capacity if we still have a space left for 1 in our cabin.

i was also told that it has been sent to 'i don't department' because the lady in the call center cannot add, once she has received confirmation only then she will be able to add.

i have been cruising with msc for a few years now. & i couldn't get an answer to my question How long will this take?