Travel changes nightmare

I booked an incorrect date for 2 travellers. I call to change the date which they glady did. I discover later that only 1 ticket was changed. I call back and they want me to pay for those changes. I don't see the fairness in that as agent was supposed to know 1 booking means both people must travel on same date. Now they have put me on hold for countless minutes hoping I hang up. Bad customer service!