My problem in my old job was due to boredom. But it was more than that - I was bored and management didn't really take heed of what was wrong despite the feedback that I have been giving at my annual performance review (where I did extremely well) and monthly meetings with the line manager. Even if I had the education and work experience with certain projects, politics ("X's department is doing that") and That's-not-your-day job-isms" got in the way. So in the end, I got bored and quit.