When you first do not succeed...try...try and even try again.
I had an momentary panic today when I tried to add an in-cabin pet reservation to our itinerary to Mexico in January.
I called CO reservations and the first CSR I got said that there was already a reservation in FC for a pet and since they have a one pet per cabin policy we could not bring our dog on board. My alternative would be to change the dates of our flight (at a cost) or fly in coach (on our paid FC ticket)...I thought of changing the dates but the next few days were also "booked" with a pet in FC.
I thought I would try again...my next call produced the exact same response. So I reluntactly agreed to change the dates of our travel but hte CSR said he couldn't help since it was an internet reservation...gave me a number to call.
I called the internet web support. They listened and said that they too see a pet in the cabin already...but....another reservation system they use showed no pet... she put me hold...and ten minutes later came back and said they she and her supervisor believe that there may be a glitch in the system and that they would go ahead and give me the in-cabin pet reservation for my original flights....they really went all out and it just pays to try as often as needed till you get a decent outcome...