I like to say it takes a minor miracle to pull off a practical test... the schedules of two people must sync up, the aircraft must be available, the weather satisfactory, no mechanical issues, no health issues. Then the applicant's endorsements and aeronautical experience requirements must be in order, the aircraft logbooks must indicate a legally airworthy aircraft, and last but not least no personal emergencies rear their heads. I've had each one of those act as showstoppers in the past.
Personal anecdote. Recently there was a young man who had been patiently waiting for a couple of weeks to get on my schedule. Timing was tight; he had to head off to college within a matter of days of the scheduled checkride. Wouldn't you know it, the morning of the practical test, the weather was beautiful, his airplane was fresh out of a 100 hour inspection and he was raring to go. But unfortunately, I was up all night with arm pain -- must have pulled my shoulder in the gym. Not only could I barely move my left arm, but I was exhausted. I had no choice but to cancel. He went off to school and we scheduled for the practical tests two weeks later, when he'd be returning back to northern NJ. The day before his return, he reached out to say that the school was quarantining due to a couple of COVID-19 cases so he would not be able to keep the appointment. The saga goes on... sometimes that's the luck of the draw.
Or, you could be like the gentleman last week who called 5 minutes after I had a next-day cancellation, and completed his activity the next morning in good weather with no issues.
