When I started playing the game I didn't realize belts had a direction and this led to a great deal of confusion a few hours in. Other Twitch streamers I've watched went through the exact same thing.
I wasn't aware of having specified a direction when placing belts, so I assumed buildings "pulled" the items they needed (and that they would pull only the items they needed from a container holding multiple item types - which also turned out to be false).
First I'd suggest adding something to address this to the early game tutorial tasks, so the player can't fail to notice this fact.
Second, it's just not visually obvious enough what the direction of a belt of building is, or when they are misaligned. I can't tell you how often I have built a storage container the wrong way round then been like "hunh?" when it wouldn't connect. I'd suggest animating chevrons when objects are being placed so it's super super obvious which direction items will flow along belts and through buildings. When you point a belt at a building and the two directions "collide", the chevrons should go red and stop moving.