Heat the pan. The best, golden, crispy dosas ONLY come out of cast iron pans. Non stick, while not healthy, also do not make golden dosas - the color is usually a bit pale. I am a bit of a purist as are many south Indians, as far as making dosas go. I might use a non stick in an emergency, but cast iron is my preferred pan. It does great in retaining heat, and is healthier.
If you use the same pan you use for rotis, there is a chance it's dried up. You will need to season it with oil, and test it with a small dosa first. You may also go over the pan with a cut up half onion to season it as you make dosas. Get it to the right temperature, and you're golden!