I ran into this problem. Honestly, it created a ton of stress as I was taking my MCAT and preparing my primary application. I knew that I would begin receiving secondary application materials within a week of my primary being released.
The professor I had a problem with was very upsetting because he was such an influential part of my education. I met with him as far back as May, he graciously agreed to write the letter, and was given all of the necessary materials. Month after month went by with nothing. Emails went answered with "It's coming shortly, sorry." Over and over again this happened. As time to submit my primary approached, I eventually left a voicemail on his office phone. I heard nothing. A week went by and I emailed again. He got back to me again and apologized. Again, another month went by. It wasn't until I laid it on the line that Interfolio finally got the letter. He wrote it over the weekend and physically handed it to the post office clerk. He profusely apologized.
I was super nervous because you obviously don't want to irritate someone with so much power over your future, but sometimes putting things in perspective is required. The reality is as follows: people with a PhD should have hopefully gotten over the passive aggressive stages of life. Hopefully they won't write a secretively bad letter. Do these people exist? Sure, but hopefully you can gauge that during the initial meeting. This is why it's so important to meet with each of your letter writers personally and professionally. If you get a wishy-washy feeling, abandon ship. If not, then the person is probably genuinely forgetful. Professors are human too. If they didn't want to help you out, you have to place confidence in them telling you so. Maintain confidence, make alternative arrangements, and keep on the pressure. I feel you. It sucks.
It can be made worse because some people have such strange social conventions. I interact professionally with people who hate email and are completely turned off by any substantive interaction via the medium. I know other professionals who somehow can't communicate via email, i.e. someone with a Master's Degree suddenly starts capitalizing everything and forgetting basic punctuation. I know others who can barely speak on a phone and get nervous. It's hard, because you are the vulnerable pre-med looking for approval. Even worse, it's one of the many steps to a successful application. A good letter will make or break an application. Like I said, I get it. Hopefully I've given you some confidence.
