That spa uses a Gecko/Spa Builders pack. It has a simple flo-though heater with a single element. That means that it is either on or not on, so the problem must be somewhere else.
Things to check: Take your filter out and soak it in a filter cleaning solution.
Be sure you are not opening the cover to check the temp - each time you do so adds close to an hour to the heating time.
Watch for flo indications: the dreaded 'three dot' signal which means that there is not enough water going through the thing: or the pressure switch is having trouble.
The Emerald web site has an excellant troubleshooting guide - it is the Gecko service manual in pdf format. Nice resource.
