Window cleaning is difficult to price, because the job only needs to take a few minutes longer than you predicted, and you've completely lost your profit margin on that job (or even worse, the job is actually costing you money to do!).
I don't think there are many window cleaners in the UK who have never made a mistake on pricing.
When it looks like the job is underpriced, I flag the job in the schedule, and I tell the team to get timings for doing that job.
When I've got at least 3 timings, I work out how much profit margin is on the job.
Then I use this technique called "Non Violent Confrontation" that I read about online and I post them a letter containing something along the lines of this:
"The fact is that we are making very little profit on your home. I'm feeling concerned because I'm needing to make a profit on the work that we do. Would you be willing to accept that the price needs to be increased to £x from 01/01/19?"
If it is a big increase then I will sometimes delay the price rise and then send a price rise letter, with my first letter about the pricing issue enclosed.