Hopper Minecarts don't commute items through them. Simple as that.
I had a dropper and on top, a Hopper Minecart. I threw items in using Q and it would just stay in the Minecart.
MC-152686
Hopper Minecarts not working
Resolved
Invalid
0
Hopper Minecarts aren't supposed to do that; they have to input into a hopper first which can then input into a dropper or whatever you want.