Jenn-Air refrigerator with large frost build up in top of freezer. Most frost issues in freezers are related to air leaks allowing warm moist air into the freezer compartment. Finding the leak is generally easy because the frost will start and protrude out from the leaking area.
If for instance the frost build up is located near the back top left corner behind the ice maker, the water inlet hose is leaking warm air into the freezer. If the frost is near the front top of the freezer the door gasket is to blame. Ive seen the complete freezer compartment full of frosty ice and that was because the door was left slightly ajar for an extended amount of time.
It is very common for bottom draw freezers to become misaligned on there rails and have the door left slightly cracked creating all kinds of frost issues. Ripped or torn door gaskets are also a issue. Look where the frost is building and you will find the solution to you problems.