Hey - I know how you feel!
It is very common for the recommenders to be late and to have to be reminded multiple times. Usually they come through but it takes a few "do you need any more info" type of emails/reminders do the job. Usually it is BETTER to go with your first choice who has had plenty of time to prep. I would attempt to contact the person one more time and if you don't hear back, you can send a "hey, I did not hear from you for a while. I am really serious about bschool and though I appreciate your effort and help with the recommendation, I will ask someone else. I value our relationship and I hope it has not put a strain on it. No, go screw yourself ahole."
The last part is optional
As an FYI:
Traditionally, college/academic references are not as encouraged due to the fact that business school is about a professional/leadership setting, and professors don't always have enough exposure in a large class to observe that. You are also in a college setting where your responsibilities are limited; consequences are muted, anyway, you get the idea.
You want to get a recommendation from someone who knows you well and can talk about you in a positive light. If they do not know you very well, the recommender needs to be briefed/helped with any of the questions they may not know answers to (some questions are purposefully insightful to see if the recommender knows you well enough).
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