For me, it has been psychology mostly.
It took a number of years to figure out a trading methodology. I started with the Ken Roberts course, which was useless outside of teaching the most basic fundamentals. Then I bought a number of other books, including the Turtle trading book. I found more success, but in the end, I had to go to the CME and CBOT and take some of thier trading courses. Those were very good and all. However it was socializing with a trader during the breaks, who stopped by to check on the class, and see how the teacher was doing that really set my system on it's path. Some of those discussion were very enlightening. I really feel that is what made the difference in the end. I found them worth the price of admission by themselves.
Once I got the system hammered out, using it in the real markets seemed to give mixed result. It was hardly the consistent wins I had papertrading. After some long retrospective analysis, I came to the conclusion the problem was execution related, based on psychological issues. I was flat out over trading, holding losses in the hopes they would recover alter, getting stopped out and all that. I actually lost my first 2 accounts this way.
I solved the problem by walking away from it for 6 months, and later going back to papertrading That took years. In that time I decided to focus on building exit signals, and building a mindset of profound detachment from the results of my trades. That eventually lead to the way I do things now.
Even now though, the biggest problems are psychological, and plain old burnout. That need to always be doing something productive, that works so well in the rest of the world, is a real account killer in the commodity world. The phrase "Try, Try again" just makes a commodity trader over trade, which racks up the losses. It really is better to blow it off and go sailing a lot of the times.