A pound to a penny says you are wshing down dirt from between the sashes
Sashes are a sod in this respect, particularly old ones
Over the years these two edges, one from the bottom sash (the top edge) and the bottom one from the top pane pick up loads of muck, if they haven't been painted properly for some years (or opened, the sashes reversed and the edges then cleaned).
Everytime water splashes up into this gap you will drag down horrible smears
Also, as this edge is not easy to putty, quite often the top edge of the glass is left exposed, yet again, over time dirt will accumulate on this top edge, and when you wash with WFP you will continually wash down this dirt.
First time, grit your teeth, aim your jets very carefully (you don't want water splashing up between the sashes) and wash & rinse that top edge as best you can.
You'll have to go back to it and do it again when the glass has dried a little on that top edge, but this time try not to wash the glass so that you also soak the top edge...
Takes a bit of practice, but you can usually get it right...but not always
Ian