Tony, you are doing nothing wrong. Some people appreciate you, some don't.
I get this daily. I am in the Emerald Isle, though, so this is the norm!
I have built my run up over the last 12 months, and, believe it or not, I retain them on the books. What I do now though, is only call or knock on the day if I have the time or inclination to do so. If I don't, and they see me and ask if I can do them today, I tell them that I have my regulars to do first and will fit them in if I can. If I run out of time, they wait till next time and I repeat the above cycle.
Believe me, my system has succeeded in getting a few custards into line!
John