DIAGNOSIS You're likely dealing with a low or empty propane tank, or the gas flow from the regulator is restricted. The Q1200's regulator has a safety feature that limits flow if it detects a leak or sudden pressure surge.
FIX First, turn all the burner knobs to OFF and disconnect the propane tank. Wait 30 seconds. Reconnect the tank hose, making sure it's threaded on straight and hand-tightened firmly. Now, slowly open the tank valve all the way. Wait another 10 seconds, then turn a burner knob to START/HI to light it. Let it run for a full minute before adjusting. If it's still weak, your tank might just be empty—try a fresh, full one.
IF THAT DIDN'T WORK The tiny holes in the burner tubes might be clogged with grease or spider webs. With the grill cold and gas OFF, remove the cooking grates and flavorizer bars. Use a stiff brush (a grill brush or old toothbrush) and a straightened paperclip to gently clear any debris from the holes along the burner. Don't enlarge the holes.
HEADS UP Never use a knife or screwdriver to clean the burner ports; you can damage the orifice and create a dangerous, uneven flame. Always check for blockages at the start of each grilling season.