Failing in med school happens for one of the following reasons:
1) Not keeping up with classes -- far and away the most common reason. Sometimes people hit a patch of burnout or get distracted by a new relationship or an old relationship gone sour, or who knows what. Point is that they don't put in the time and pay the price.
2) Mental illness/substance abuse -- close second and somewhat tied in with #1, but deserves its own category. These people also get behind but the etiology is obviously different and dealing with it is different too.
3) Inadequate intelligence - rare but it happens. The admissions process prevents this most of the time, but there are some truly soft admits that just don't have the chops no matter how hard they work.
4) Physical illness/traumatic life event -- probably the least common cause of failure because the overwhelming majority of people who face something like this will just take a leave of absence. I've never known our dean's office to be anything but totally understanding and helpful when major things happen.
The first 2 being the most common, it also makes sense why these people don't show up for tutoring and extra help. I know our dean's office mandates repeating students go to tutoring and have it documented (I've done a lot of tutoring for these folks).
Whatever you do, med school is not a time to suffer in silence and go it alone. Reach out for help early and often. Reach out for help even if you don't think you need it. Worst case scenario you end up overprepared.