Problems Adding certain films to List

Hello I noticed that when I am trying to add certain items to a list I want to create they do not show up in the search bar of the list creation page. However, I search for them on Listal and even at times place their IMDB link in the search tab to make sure they have been added. But, then I find out they are already on Listal but for whatever reason they still do not appear when I search for them when making a list is there anything I am doing wrong? Let me know when you can :) and thank you!