Sash windows can be awkward, but the biggest problem is when their frames have oxidised.
You also have to take care on the lower sash not to spray water up into the gap between the 2 sashes.
But oxidisation can be a problem on any window, even mild oxidisation can cause spotting.
Fortunately it is only a small percentage of painted or aluminium windows that have this problem.
Changing long held existing customers over to WFP won't be easy, you will probably lose a few, but you should aslo begin picking up new ones because you DO have WFP.
There is a steep learning curve needed though, and that can be part of the problem
It takes a while to become good at what you are doing.
I've been going for 12 months with my system, for quite some time now I would say that in most cases, even on a first time clean I will do a top job, something you will struggle do in the first few weeks of first time cleans.....and of course everything for a month is a first time clean
Ian