As stated above if, not used for a while may take a couple of minutes holding the button in until the gas gets to the burner. If you have a pizzo-electric ignitor I have had sometimes difficulty igniting the burner with it and wound up using a standard grill lighter. In one case a spider had a small web insider the burner tube which prevented ignition - Cleaned out the spider web and it worked like a charm. Some of the fancier units have electronic spark ignitors and in that case it could be no power or a bad electronic module. Depending what system is on your unit it may be a relatively easy fix or perhaps off to the RV shop.