Your dog definitely doesn't understand what is expected of him so retraining is a good idea. However, if it didn't work the first time, perhaps you need help. I strongly suggest enrolling in a positive obedience class. The social interaction of other dogs along with a qualified teacher offering help and suggestions along the way are a win win situation in training.

In practice at home, you need to make sure your dog understands every detail completely before moving forward to other commands. If you move too fast the dog will be overwhelmed. There are many wonderful training sites online. I highly recommend Karen Pryor's clicker training site. Clicker training makes training the dog faster and more accurate.