Not having a well-seasoned Tawa is a big reason Dosa gets stuck, especially around the edges where the seasoning flakes off. So, when you try to release it with a spatula, those edges can be a real pain, while the rest comes off pretty smoothly.
Cast iron is not a good conductor of heat. Cast iron Tawas often have a temperature difference of 60-90 °C between the centre and the edges. So, when the flame is too low, the edges barely get heated. As a result, the batter doesn't crisp enough to smoothly release from the Tawa.
If you use raw rice or if you add too much Poha while grinding the batter, your dosa may get stuck and end up in a mess. So, avoid raw rice and instead use idli, dosa or parboiled rice for making Dosa batter. This is also good for making soft and fluffy idlis.
Unlike nonstick Tawas, cast iron Tawa needs sufficient oil so that Dosa releases without much effort. So, apply a thin layer of oil and ensure that the Tawa is warm enough before you spread the batter. Also, after spreading the batter, spread some ghee or oil over it, especially on the edges.
Poorly constructed cast iron may have microscopic cracks and air bubbles that prevent proper conduction of heat. In such cases, no matter how well you season, the results won't be ideal.
Often people use the same Tawa for making Dosa and Chapati. The burnt bits from the chapati may damage the seasoning of the Tawa and as a result, your dosa won't come out well. So, always keep separate Tawa for Dosa and Chapathi.