If you are new to window cleaning then you will be more prone to pick up other cleaners rejects. As very few messers change, they will tend to become your rejects as well.
In the early days you have to proactively build your business, which means collecting the debts as well. In time you will get to know your genuine customers and the time wasters. Although we now have a two clean rule, this doesn't extend to new customers. They also have to build their credability with us. Someone on here started the 3 c's - canvass, clean, cull. You will have to decide how much nonsense you will take from new customers whilst you are building your business.
In time you will be able to afford to 'let them go' when you replace them with another.
Your customer base will always be 'fluid' - rental customers move, die, sell up etc. most that you have little control of. Each time a customer messes around it costs you money - cancelled cleans, locked rear gates when they promised to leave them open, dogs muck, delayed payment, repeated collection calls - so don't hold onto them too long.
