Hi, well this could be a number of things unfortunately, when this unit isn't heating, it maybe the Element itself, the temperature sensor, the thermostat, or the main CCU. There is simply no way to tell without actually performing the troubleshooting steps to locate and isolate the problem with the view to repair.
Above is a link to obtain a service manual, below is a link for parts and a parts diagram.
If you wish to DIY, then you will need to obtain a service manual and using this perform the troubleshooting procedures to locate and isolate the faulty parts, and also to be able to use the parts numbers to order replacements.
Should you not wish to DIY, then the very best thing you can do is contact you Whirlpool local or head office and ask for a referral to an authorised service center. Now when in face to face contact only first ask for a "Quote" as to the costings of the prospective repair, this way you can then make up your mind as to what to do repair or replace? At the very least you will also then know eaxctly whats wrong and how much it will cost and go from there.