nancydrewmaster there is a multitude of possiblities. Excessive lint inside the cabinet and burner compartment of the machine is one of many. If the burner area inside the cabinet is excessively linted, the burner flames will be yellowed (it should ne nice and blue). I would clean out the area under the filter, vacuum all ust and lint out of the burner assembly and motor. After that clean out the vent and make sure its clear and all as straight and clean as possible. Be sure to clean out the area around where the burner meets the gas valve. That area must be free of all lint, dust, etc. Cleaning out the entire interior may solve the problem.
There are of course other reasons like low gas pressure, air gap adjusted wrong, poor air flow, etc. that can cause this. Start with the easy stuff. It might help if you give us the proper make and model of your dryer so we can try and be more specific.