When you put the lemon detergent in with some white distilled vinegar into the boiling water to soak, it should ideally be boiled up and left for a few hours to soak. Also the baking soda is excellent. Sometimes, I use St. Moritz oven cleaner and leave for a few hours.
For the coffee/tea mugs that are very discolored you can put a few drops of bleach into the cup with some lemon detergent and that makes them very clean. Rinse well and put in boiling water and they are like brand new.