Well, if you have noticeable lag issues, then that can really hurt your game - I know this from experience. Not sure if you have lag? Well, walk from one side of the lobby to the other...if your avatar walks smoothly all the way across, then you aren't showing obvious signs of lag. If in game, check the flight path of any given firework - is it smooth or does it hiccup. When the star appears in the firework, does it rotate smoothly or does it quickly disappear with almost no rotation?
If lag isn't the issue, then go for flawless hits. If you don't get any combinations, but get all flawlesses, your score should still be halfway decent. I usually tell people to not worry about the combinations if they haven't mastered getting flawless hits. One thing at a time...
Also, there are websites dedicated to testing the latency of your connection. I recommend going to one of them just to see what they say about your connection. The one I frequent most is
http://www.speedtest.net.
I wish you luck and improvement in your fireworks games.