I can do well in sit n go and mtt tourneys, but suck at ring games?

In the sit n goes i have played I've placed: 1 2 2 1 3 3 4 5 7 9 4 3 out of 9. The one MTT i've played was out of 1879, and I placed 6th (sadly, it was a freeroll. yay $3!!). I've just started out online and this has been bothering me big time. I just dump all my winnings from SnGs into live game player's hands.