The city and county roads departments I deal with are pretty responsive to actual damage to the road, potholes and such, even when they are in the bike lane. When it's just a cleaning issue, they tell me the street sweeper schedule and that's it, even though in winter months the street sweepers tend to skip the bike lanes.

I'd suggest that you need to frame this problem as a structural failure -- the retaining wall isn't holding back the dirt/sand/etc it is meant to contain. If it is interpreted as a cleaning issue it will keep coming back on a regular basis.