It also has to be said that some panes of glass WILL ALWAYS SPOT!
Even on the same house with apparently the same windows, sometimes you'll get a particular pane that will spot every time
And just because a window has UPVC frames it doesn't mean it it won't oxidise either, so give any problem windows the finger test (wet finger and rub on frame, if your finger comes away with even a hint of the colour of the frame on it, then the frame has began to oxidise)
Sometimes it may be the seals between frame and glass that causes the problem.
So it isn't necessarily always the fault of poor technique!..........Unless of course it is happening on every window you do
If that is the case then you just have to keep practising until you get a technique that suits you and also does a top job.
Each of us have slightly different ways of going about using WFP, have a go at each of the methods, you'll soon latch on to the one that suits you best...
Ian