You should only use the command once you know for absolute that your dog is going to offer the behavior after saying it.
So if he is stopping and looking at you after you say it - it hasn't been associated yet. You still need to practice the command + hand signal for many more sessions, maybe even a week. (Depending on how many sessions per day).
An important thing to remember is that when you give your command - you should not give ANY body language. You should be standing still. Many people make the mistake of saying their command AT THE SAME TIME that they give their hand signal. This will never work. There has to be a 1 second gap between the two.
So if you train the command + hand signal often enough, you will see your dog wanting to roll over just before you give your signal (or right after the command). This is the time that you can give him a jackpot (give him many treats) to reinforce him for going after the command.
Another thing to remember is that the tone of voice of your command should always remain the same. Some people tend to change tone of voice depending on how they feel, this will confuse your dog. So pick a tone of voice you want to use and stick with it throughout your training. :dogsmile:
If you are going to train multiple behaviors, I suggest that you train them separately. In other words, train the roll over for a few minutes and then switch location and train another behavior.
Hope this helps,