The doorknob is the moment just before a door opens, so dream books read it as choice and hesitation. A knob that will not turn is taken as an opportunity that feels out of reach.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.