I start scheduling more than a week ahead of the date I want to session and only contact one Domme at a time. If I don't hear in two or three days I either resend it or move onto my second choice. The later is extremely rare. The second email to the same Domme has almost always worked.

The message you compose makes a difference. It should be brief, listing some of what you want to do in session, and your proposed date(s) and time(s).