Look for bad spot in door gasket. If air leaks in, humidity condenses out on fins and freezes. Try the dollar bill test. Close a dollar bill in door and verify that there is drag when you try to pull it out. Make sure there isn't something in the door or on a lower shelf that is holding the bottom of the door open slightly even when the latch is latched.
Some icing on fins is normal. Mine usually takes 4 - 6 weeks to ice up the fins to the point that they need to be thawed out. Only saw them ice up in 1 day when grandkids left the door slightly ajar.