I spent half an hour there trying to get a few outstanding in, don't really have a problem with it either though. I do the job, if they are in great, there booking is arranging down to the hour so I do ask that they try be in, but if not, I appreciate things can pop-up so I do the work and post a note asking them to pay by cheque to call to arrange a time they will be in. Most adhere to it but you do get a couple that just ignore it.
When I get someone who is never in, doesn't call or send payment, they get two bookings and then they are dropped. Even if they don't do it consecutively but are in a habit of messing me about, I just wont go back, its just not worth the bother with some, they can end up costing you rather than you earning anything.