"We already filed an extension when you did not show up.  When would you like to come in next week?  (Or next month, if you're taking some time off.)  The sooner you file the better, because you will owe interest and maybe penalty for any balance due.  By the way, the fee for filing extensions is $100."

I had one no-show this tax season, but I know why.  My scheduler thought they wanted February 25, but they meant March 25.  It used to happen once or twice a year, even though I emphasized the importance of repeating the month a couple of times just to make sure we were, so to speak, on the same page.  Then the past couple years, we didn't do appointments.  This year, many of those who demanded appointments in the past, figured out it was easier to mail or drop off their stuff.