Hopper Loop starts skipping Hoppers

I created a loop of 6 hoppers, all attached to the one next to them and put one item in this loop. Next to each hopper is a comparator to display where the item currently is.
A block with redstone on it is on top of each hopper to start and stop them giving the item around. (See screenshot for setup)

The goal was to create a simple and compact clock, which can be locked on one position by a lever.
This works for some time, but after a few uses, it starts randomly skipping hoppers until the item only gets swapped between two hoppers, even if they aren't next to each other.
Re-placing the hoppers will temporarily fix this.