I found the manual and see what is going on - you've probably been using your smoker in grill mode and, as Uncle Bob noted, the heating element might be weak - or - you are pulling the plug as soon as you pull the food off the grill allowing the fats to congeal on the rocks.
I can think of three possible options:
1) Put the lid on in grill mode and plug it in for an hour or two to see if that burns the fat off.
2) Remove the heating element and in a safe area - sprinkle a little charcoal lighter on the lava rocks - wait a couple of minutes - sprinkle a little more - wait a couple of minutes and then light.
3) Put the lava rocks in a pot and cover with 6-8 inches of water - bring to a boil and simmer for an hour or two. Turn the heat off and allow to cool - skim the fat off the top of the water and then drain, rinse, and spread the rocks out to dry before using them again.
Or ... you could buy a new bag of rocks.