In general, the freeze dryer will heat the food slowly throughout the dry cycle. It will only reach full shelf temperature at the end of the dry cycle. This is normal operation. 

If the heat racks never turn on at all during the dry cycle and the food is not done after an excessive amount of time, then it is possibly a vacuum leak, or the vacuum pump is worn and not able to pull enough vacuum any longer. The machine operates such that it will only heat the trays until it reaches the set pressure. As the trays heat they cause more water vapor to read as pressure in the chamber. If there is a vacuum leak, or if the pumps ultimate vacuum is no longer adequate, then the machine will never turn the heat racks on because it is already at or above the set pressure. To test this, start by doing an ultimate vacuum test. Go to settings, then manual hardware control. Then turn on the vacuum pump and the refrigeration together. Let it run for 1 hour and then check the pressure reading. If the unit is able to get below 500 mtorr the the pump is fine and there is not a vacuum leak. If the unit is above 500 mtorr, but below 1400 mtorr, start by raising the pressure on the recipes by a few hundred mtorr. It is likely the pump is worn a little. By raising the mtorr on the recipes it will allow you to continue to freeze dry. If the ultimate vacuum is above 1400 mtorr it is likely the pump is worn and no longer able to pull a vacuum. It is possible there is a leak, but that is a much smaller possibility.