Hoppers only sometimes take items

I had a video, but the file limit got reached so I have to write it out.

 

I had a minecart hopper that runs over a hopper, and it never picks up the items from the minecart hopper. Also, when I throw items into the hopper doesn't pick it up so I'm a bit confused. Usually, the hopper would pick it up, but it doesn't, and this breaks many contraptions which is super annoying.