We have a supposedly frost-free freezer which mostly works as designed: the
cooling fins on the heat-exchanger remain clear of ice. But several times
over the last year, we have found the reported temperature increasing and a
lot of ice around the heat-exchanger, so we have to take everything out
(thank goodness for a spare freezer) and defrost/re-freeze.

There is no sign of obvious blockage in the pipe that drains the
heat-exchanger to a tray above the motor/compressor.

It doesn't seem to happen at any particular time of year, and as far as we
are aware, we don't store more/less food in there or open the door more at
some times of year than another.

I'm intrigued what could cause a freezer to keep itself frost-free for
several months (with no visible gradual build-op of ice) and then suddenly
ice up.