Jet Airways — seat availability for issued ticket

I have booked flight through jet airways site, the carrier was etihad airways and the date was on 2215 hrs 6th dec auh to ccj, when i came to counter the they are saying the flight is fully booked and my wife cannot travel with ticket issued flight, they have to wait for the another flight. This is stupid thing that confirmed ticket holder cannot go with the flight, etihad airways give the another ticket with complimentary voucher and this complimentary voucher issued with out asking us. Understand that etihad counter they are not liable any thing issued through jet airways site. Question is why they are saying we partner airlines and doing with people.