Bare in mind that works on a ranking base.
If you are winning against drivers with very low ranking, you will collect fewer points, progress is slow.
I was fast to get out of "E" without winning any race, stuck in "D" since then, despite winning two races on "D" level.
Once, for example, on a race whit DR "D" and "C" paired players, the guy who won was a "C" level" and it was is first win, according with the screen post race. And the guy was pretty fast. I guess he had "bad luck" being paired with strong rivals, didn't had much chances of winning races, but good finishes against fierce opposition granted him good points.
PS.: in my experience, sometime ago I had a couple of days off, realized in the mornings on weekdays, the opposition level is pretty low on european region, easy to get good results (in my case that didn't do much for my DR rating either), I guess fewer players and younger ones online.