Hoppers sometimes don't follow their set rules

Sometimes when a hopper is facing into a chest in front of it (horizontally), and you have a hopper below it, the expected outcome is that the items flow to the bottom hopper, this is usually the case although sometimes it goes to the chest. I assume this is a bug as it hasn't worked like that before, and it seems inconsistent.
(It may take a while of inputting items through the hoppers, as well it may be due to lag.)