Tying to work with existing reservation to view "change seat", "redeem upgrade", and similar functions. About 3/4 of time get the message below. Just consistency inconsistent

"We’re sorry, but united.com was unable to complete your request. Please try later or contact united.com Support in the U.S. and Canada at 1-800-396-1751; the U.K., phone 0800 028 5003; elsewhere use the local phone number".