If dosa batter stick to the tawa, cut an onion into half.
Clean tawa with it and little oil.
If this still does not help, heat tawa empty very well till almost fuming. Sprinkle salt all over. Switch off the gas. rub with a thick cloth so that your hand should not burn, remove the salt. Let tawa cool.
Then reheat and make dosas. Dosas will not stick.