Can't get rid of garbage in a dense city
Solution 1:
When garbage is collected by visiting volunteers, it is not counted in the summary numbers.
Review the garbage collection dataview. If there is no garbage in any building, your garbage is collected. There is no benefit to adding more service vehicles in this situation.
Also, with update 7's change to pathfinding for service vehicles, garbage trucks are much more efficient than in the past. You may find that 3 garages (6 trucks) can pick up all the garbage in a city that size.