If your propane tank is mounted in a movable drawer, your supply hose from tank to grill has probably kinked. Jenn-Aire and Ducane is notorius for this but they say it is not a problem. I'm replacing with a longer and more flexible hose.

There is either an obstruction in the gas pipes like a spider web or something or your valve is not opening up all the way. It also could be a bad attachment fitting or not seated properly. I hope this helps.