Your own inexperience is also a major factor, when I now clean a house for the first time, and in particular if it is UPVC windows (new ones) then I can almost guarantee they will come up absolutely immaculate.
Keep a fairly high flow rate (as you gain experience you will find you can reduce it)
Being as you are WFP newbies, wash the frames thoroughly, and then the glass, take your time.
Go all around the upstairs windows first, wash the windows as thoroughly as you can, working top to bottom.
Once you have finished, go back to the first window and clean just the glass only, go over all the windows on the upstairs again.
Then do the same on the downstairs windows.
Don't worry about how much water you are using, and don't worry about how long it is taking you.
In my own personal opinion you should also clean out your brush head regularly too.
A little detergent in a bucket of pure water, preferably one of the specialist window cleaning detergents, Unger liquid of GG3 for instance.
I really do think that keeping the bristles on the brush cleaned make a big difference where spotting is concerned.
This won't be the problems that you are suffering from, this is more likely down to your inexperience and lack of technique. It really is a bit of a steep learning curve.
I'm going to do a separate thread on my musings with regards to contanimated bristles, I won't go into that here.
But keep that water running all the time you are on the windows!!
As a sort of general guideline you can expect to use as much as 20-25l on a first time clean on a standard semi, more if there is a conservatory.
As you gain experience that usage will drop, but don't worry about it in the first few weeks.
Ian