Every time I think I've got it licked, someone busts a big one on me. I have learned from doing big runbacks myself and being a pest:
1. Don't be afraid to put big guys on your kickoff team to distract blockers. Most big disasters happen because the coverage team is overpowered. In RL a returner would simply dance around a big, slow guy, but of course this isn't RL. The conventional wisdom is to avoid guys like that, but the conventional wisdom isn't very wise.
2. Put different kinds of players next to each other to make them harder to block. Lots of owners rely on just one designated playmaker on the coverage team, and if that one guy gets blocked, or if he whiffs, then the coverage team is screwed. I think it's better to take lots of little shots than just one big shot.
It seems to me that your problem comes from putting your best guys all together so that they can get blocked or avoided. Try spacing them out and see it that works better for you.