I know this is almost the anniversary for this post, but I'm adding a quick note anyway.
Frost is fluffy and contains a LOT of air. Wool is fluffy and contains a lot of air. Fiberglass insulation is fluffy and contains a lot of air.
Air is insulation. Anything that is fluffy and contains air will insulate. Just like your wife's last perm.
A good block of frost up in the compartment venting will cut off all or almost all air circulation. This frost is not always obvious.
If you have a bad problem, unload the unit, unplug it, and leave it w/ open doors for 24 hrs.
Warm air, humid air, and little kids will make your frost problem worse.
I've collected a lot of service calls by explaining this. And lost more than a few by explaining this over the phone.