Quote from Cutten:
I found that it was necessary to do "practise", as opposed to just play real games. Also, review your games intensely for errors, possible improvements etc. If you look at other fields, it is directed training, coaching, and practise which creates great skill. A champion boxer may only fight for 20 hours during the entire course of his career. The other 20,000+ hours are spent in the gym training and exercising. Top players might spend months, 14 hours a day preparing for a world championship match, that only lasts 100 hours of playing time.
IMO, most people who get "stuck" spend much more time playing than training.
The second factor is that it becomes harder to learn as you get older. Most people who became very skilled, saw most of their improvement at a young age. It's rare to see someone who was grade 2000 at age 30, suddenly improve and become a grandmaster by 35 or 40. It's like learning a language, much easier to do when you are growing up.