Sorry for the delay. If the dryer isn't getting hot and the heater and drum thermostat has been replaced already then this just leaves the timer.
As theres only 3 things in the heater circuit and that's the heater, thermostat and the timer.
So if its still not heating up then im afraid the main timer unit is the fault and will need to be replaced to cure this.
If you need anymore info just get back to me.