1- not sticking to it (if it's keto/low carb or fasting)
2- if it's "calorie counting" it will fail at some point due to the lowering of basal metabolic rate.
3- cheating on the diet (eating things that are not part of the plan)
4- hormones can make a diet ineffective (too much insulin, not enough HGH etc)
5- medications can make a diet ineffective
6- permanent stress can make a diet ineffective
7- not enough sleep
8- too much alcohol