Mary Ruth good question, and Home Acre Mum, good response.
To add my 2 cents . . . I normally make 1-3 US Gallon milk batches and the larger volumes do definitely hold their temp better, for smaller volumes, I just check the temp every 15-30 min and if required give it a 20-20 second burst of gas as it's already sitting on our stove. Double boiler - water bath would be better, I'm just too lazy. Some people use sink with hot water as their bath.
On your other batches where you didn't get a good curd set, feel free to post your method, ingredients, and results, there are several people here who could try and identify any causes.
Hope you don;t mind but I've changed your post's Subject so thta more user friendly to others with similar question.