You definitely should have had the temperature up. You want the mixture of oil/water/bud to be at 100°C (boiling point of water). You don't have to worry about the THC vaporising off, because it doesn't vape until 190°C or higher. Just as long as you keep some water in the pan, the entire mixture will never get higher than the boiling point of water. Also you need to simmer it long enough for all the cells of the plant (that you ground up, right?) to break and release the goodies.