I would suggest writing a formal complaint/letter to customer care:
http://www.united.com/web/en-US/cont...r/default.aspx
They have more authority to make things right than a phone agent. Also, perhaps next time you would be better served to stay an extra night in FRA, and take the same flight the next day in order to get the same service you had ticketed for previously. Assuming that is a viable option.