I'd say the top three would be:
- Not enough calories
- Not enough sleep
- Not enough consistency in workouts.
1) A lot of people wanna get big, but they try to get big while "not gaining any fat." It's just about impossible to get bigger muscles without also gaining some fat. Gaining an inch around your gut every week is excessive, but you won't grow if you try to stay cut at the same time.
2) Sleep. Getting too little sleep at night hinders recuperation and makes it difficult to properly recuperate from one workout to the next. It also diminishes your performance at work and everywhere else. You'll also catch colds more often. Shuteye is good.
3) Consistency. I don't train at a commercial gym anymore, but when I did, I remember a lot of people who'd come in everyday for a week, then 2-3 days the next week, then 1 day a week after that. They'd repeat that pattern over the course of the year, looking pretty much the same all year round.
Or you'd have people trying a new workout method every week, never taking the time to really learn the movements and make any progress on a program. One week it's Arnold's program. The next it's 100 reps; then 6/20s; then pre-exhaust, 21s... Probably was Weider's Confusion Principle. I don't know if their muscles were confused, but they sure weren't growing.