you should have asked if the miles you were buying would post immediately or not. If you did ask--or if the agent alluded to this being the case and then it was NOT, then you shoudl write them and ask for help. You may not get far, but maybe if that is the case, they can refund the miles purchase to you. They should if that is the case.
I guess you did this on AA? I could be wrong but they are the only ones who I know of who will let you make a res when short of award level. If it is AA, they have been able to extend reservations for longer periods in my experience, but when they do fall out, then yes, they do not go back into AWARD inventory. Though this stinks, I think one reason is because they can re-adjust the number of alotted free seats if new people wish to buy tickets. they would rather sell them than give them away.
I dont like it either.
As for the fact that AA has been known to extend and re-extend award reservations, it happened to me when I had set up 4 Hawaii awards using 35k miles each, and one award that required 25K plus money. This award exsited for a short while and so I took it because I had a voucher with that airline. I had been instructed to send it in with a coupon that would deduct the 25k, and also my credit card info to deduct any remaining money due. So the 5th person traveling to Hawaii would be using 25k+money+ voucher and the other 4 were just using my miles.
Since the airline had not yet received the mailing when the reservation was about to expire for all 5 seats, they were able to extend it for me twice... for a total of about 70 days!!! It was good, but that was in the winter of 2001/02... Long ago, if you are considering changes in the airline industry (or the price of gas!)
Good luck! YOu now at least have enough for the NEXT trip.