Yup, some good advice here. I have built my runs from scratch by good old fashioned door knocking. I didn't take too much notice if the windows were already clean. It may be that the householder does them themselves, and was waiting or looking for a window cleaner to call.
I am in the south west of Ireland, and have knocked around 5000 doors in my locality. I currently have 378 customers of varying regularities. It has taken me 20 months to get to this stage, and I reckon it will take me another year to get to where I need to be.
It is definitely tougher going than it was a few years ago, but with enough determination and motivation it is still possible to build a decent run.
To the OP, you have made a good start. Stick with it, and as has been suggested, try canvassing at times when people are more likely to be in. The more doors you knock, the closer you will get to reaching your target.
Best wishes,
John