It can be hard when your building a round up and you feel caught between a rock n a hard place. In one hand you can't wait to get rid of them, but in the other you need them
My advice would be to press forward as you mean to go on. Don't wait until you are at full capacity before you nip this in the bud. It's a confidence thing!
What's not convenient for them? If you feel these are poor excuses then set your stall out. Simply let them know the situation. Ie:
"Unfortunately I schedule my work per day, per week and I can appreciate this may not be convenient for you but you will be pleased to know you don't have to be present when I'm cleaning your windows. However I am happy to reschedule for 4wks time when Im back in the area, but due to extra work required there would be a 25% sur charge added.
Or I can stick to schedule and clean tomorrow at normal charge."
Ive found in the past, that this works as they say OK, just do it as usual then.