There are many reasons why the culture medium does not coagulate, such as insufficient agar, low pH value and high content of inorganic salts or activated carbon. The phenomenon you mentioned may be caused by the low pH value, and the culture medium will become lower every time it is heated. If it is too low, it will not condense. Water, hormones or inorganic salts in the culture medium make the culture medium have a low pH value, but it cannot coagulate after direct sterilization. After preparing the culture medium, you can try to raise the pH value a little.