It’s much harder to stay on top of your GTD if the system isn’t working for you. GTD works, so don’t blame the system. There are a number of small things that can go wrong, however, if you aren’t careful.
Exercise regular reviews. If you aren’t reviewing, you aren’t Getting Things Done. I review my action list a minimum of once a day. I review my action list when I think to myself “what should I be doing?” I review my Someday/Maybe list at the first of every month, and whenever I’m feeling day-dreamy. I review my project actions every time my brain switches contexts to that project.