I have converted my dryer from propane back to gas and now it doesn't heat well. What is going on

The most likely issue is that the gas orifice is still sized for Propane. A propane orifice is smaller than a natural gas orifice because propane has a higher BTU content than natural gas and thus requires a smaller volume of gas to provide the same amount of heat. If you change the gas orifice back to one required for natural gas there should be the correct amount of gas going to the burner. This is something you will need to ask your Gas Fitter to do as most areas require all gas work to be performed by a certified Technician, Good Luck.