I bought a round from a trad window cleasner weho had been doing alot of them for 20 years+, ther longest for 32 years.
Some would just not accept the pole, even thought they said the windows were fine, lost about 15-20% eventually, but had worked them all enought o make my money back on what I had paid for them.
Always factor in and accept there will be loses, then it won't bother you when they do happen.