The main reason for blondies not baking up properly is the overmixing of eggs. If you beat the eggs too much, they will trap too much air and cause the mixture to sink during baking. If you have not allowed ample time for baking, it may also cause the center to sink because it did not have enough time to rise. Always check your blondies from the center with the help of a toothpick to know if they are ready or not. If the toothpick comes out clean, the blondies are ready.