One thing I often observe in my teaching at two colleges and privately:  students don't know how to practice (ok, it's part of the bigger problem: they don't make time to practice or maximize the time they have).

This article has some great ideas.  

http://www.npr.org/sections/deceptivecadence/2013/09/03/216906386/10-easy-ways-to-optimize-your-music-practice