Occasionally the hold option does not show up as an option when I am making reservations. One of my guesses is that although I was logged in, the website has lost track of who I was. Sometimes when I rebuild the itinerary in a different window after making absolutely sure I am recognized for being me, the hold option reappears.

I have also had the frustration of my itineraries not working and not being able to book a reservation already on hold and having to start over.

Just on of the many annoyances of united.com. However I would still rather have the option to try to do it myself online than rely on the CSRs that are only right 1/3 rd of the time.