There are a number of possible reasons for not being able to place a hold on a particular item:

  1. You have a children's or young adult card and the item you want can only be borrowed on an adult card.
  2. The item you are trying to place a hold on is reference only (i.e. can only be consulted in the library) or can't be reserved for some reason.
  3. You already have 12 reserves and so you cannot reserve any more.