Thank you.

Obviously, there is no way I can safely diagnose an issue like this without looking at it, but if you are just looking for some ideas I can offer up a few basic ideas. However, if you are unsure or uncomfortable with working on gas appliances, be sure to contact the manufacturer.
With that said, there are a few things that might affect the flame height. First off is this a new grill? If so make sure it has the correct orifices and regulator. Natural gas orifices are bigger and if they were used with propane you would get much larger flames. It would also be helpful to know if the flames are blue or do they have a lot of yellow. Blue, unstable flames are a sign of too much air. Yellow flames are a sign of not enough air. With this in mind, you may need to try adjusting the air vents on each burner until you get a nice, blue stable flame. Another possibility is your regulator could have an issue and is supplying too much pressure. I hope this gives you a few things to consider. Let us know how you make out.