As a general rule if someone tells me not this month I tell them I don't work on that basis. If they don't agree they are history.
Now - if I have a really top dollar custy say an easy £25 half hour job then I might just cut them some slack if they have a half decent reason once a year. But the following are instant drops:-
"They're not dirty"
"I haven't got any money" (I accept online and cheques in the post so shouldn't be an issue)
"Not this time"
"It looks like rain"
"My dog died/I'm not feeling well/the parrot goes mental/my lazy sod of a son is still in bed"
I usually consider:-
"Putty's still soft from decorators"
"I'm having new windows in a couple of weeks"
"Decorators' are painting them next week"
"My husband has just died and they're coming to take him away today"
If they phone in advance I usually consider a one off "not this time".
My "compass" gravitates to instant drop and so my custies are well trained.