Needs some cleaning! Take the burners out, remove the cotter pins that are behind and beneath the BBQ and shake them or air gun them and get all the dirt out of the burners. Keep in mind that if your burners are bowed it is time for new ones.
If your grill is brand new out the box and your running L.P gas, then you might have a N.G Unit

1. Check the line. There is a chance that there is foam at the entrance of your gas line located in the back right corner of the grill. Might have a fouled regulator. There might be a gas leak plugging your regulator for safety.

1. Regulator might be restricting too much gas.

1. Valve might need replacing. Regulator might be restricting too much gas.

1. Burners either need to be cleaned, or there is something disturbing the flow of the gas making the gas travel back towards the valve and with the ignition igniting the gas you are getting fire right by the knobs.

1. Check all connections to the grill if there are no leaks then there is a chance that there is actually a leak right underneath the grill. To check it is easy, get in front of the grill and look at the valves from underneath. There is one screw that holds each valve into place. Tighten each one snuggly, this should be the solution.

1. 99% of the time Valves need to be replaced, the ignitions system has failed.