My hoppers are depositing into droppers fine, but i repeated that method until i got to the y level 15 to deposit my items into an underground base. My items stop in the 2 to last hopper or dropper. The items do not go into the last hopper and dropper.
Hoppers work regardless of their Y level. Something else is breaking your machine. Although this may not help, here are some things that could be at fault:
1) Hoppers will not work if they are powered by redstone. If there's redstone nearby, turn it off or take it out. If it needs to be on all the time, try rewiring it so it doesn't touch the hoppers.
2) Make sure your hoppers are all pointing the right direction. A hopper facing the wrong way might not move the items.
I hope this helps you solve your problem.