The heating tube on the household oven, at first the heating tube is red and can be heated, but after heating for a while, it will no longer heat, what's the matter?

It should be normal, related to the adjusted temperature, when the temperature reaches the set regulated temperature, the thermostat will cut off the power to stop heating. When the temperature is lower than the adjusted temperature, the thermostat will turn on the power supply to continue heating, and so on repeatedly.