Orbita/Spirit did the same thing to us, for four reservations from Orlando to Grand Cayman, and we had to give up our exchange to Morritt's Grand in January.
Both blamed it on each other. Orbitz said not not use Spirit, that they had complaints from others, and Spirit said that's what happens when you use travel agents, that they sometimes book reservations when there is no availability.
I caught the mistake because I check our credit card transactions online occasionally, and I could see a charge from Orbitz but none from Spirit.
That reminds me, I just had a spate of cancelling and rebooking another reservation, the one back from Orlando to here, so I need to check our CC to see if it is OK.