And the ultimate slope, a vertical drop, doesn't seem to be a clog
problem due to settling.

Vertical has gravity working for it along with no drag. A steeper than
appropriate drop could certainly have issues with solids settling on it.
The flow ends up being 'too quick' and moves over the solids instead of just
pooling up slightly behind it and forcing it along. I think there's also
the issue with how solids might build up over time. I'm sure there's a more
scientific explanation out there.