Many worms like to remain in the lower trays of stacking tray systems. In fact they are unaware of the tray system really as there are holes in each tray so it's fair to say it is not an exact science. Most worms will move up. Not all will.
Realistically the whole food thing is not really understood by the worms either. They actually munch on the microbes that break down what we call food. In the process the also ingest the "food." If there are still microbes in the lower tray (hint: there are LOTS of them!), the worms will still find plenty to eat down there.
So it comes down to some worms preferring to move up out of their waste in search of decaying organic matter to munch on, while others are content to stay in their own waste looking for microbes there, usually this is newly hatched or younger worms.
The main learning (as usual) is for us humans. We need to ensure that the kit is not waterlogged, not too hot or too cold, has plenty of air circulating and a pH level of around neutral. The clue as to whether or not your worms are working well are a: Can you see flecks of brown compost - and particularly up the sides of the tray you are adding waste into (if so obviously they are working there) b: If the waste is just rotting is it too wet (add shredded cardboard) c: Is it smelly (add lime mix) d: Are they working to the top layer (add a moisture mat) e: Do the worms need a boost? The best additive for this is to add a small amount of rock dust which really encourages growth and a healthy population. (Adding an extra tray helps as well as this gives them more space to work in and so more worm capsules tend to hatch out).