This is normally due to too much water being added to the pot. Silverbeet leaves are tender and cook quickly. The water is there mostly to help the rice cook and should be filled just above the top layer in line with the bottom of the plate which is covering the contents of the pot.  Another reason was the silverbeet leaves contained too many holes or were not rolled up securely. The other reason could be the pot was boiled for too long and the heat was not reduced earlier.