It happens to me too, lots of times, and for some of them I've got a theory:
If you are reading to someone, that person is asking something, there ares things that the person wants to see revealed, and are other things, important information that he / she is occulting... normally that goes with the first card in the spread, sometimes in the middle etc...
Try to figure out what he / she is hidding, from you, that is not supposed for you to guess, it's a key information for your reading, the key of the context, the salt, the pepper of it and it changes it all.
Sometimes when I pull a card and... nothing... I ask: is there anything I should know to contextualize this reading...? Often there is
If you are reading for your self, stop and think all over the situation and reformulate your question.
What do you guyes think of this??