I had this problem in a summer place that did not have a forced air
heating system with an air filter. Lint would clog the line. I
invested in a roll of fairly heavy stranded insulated wire at the
local hardware store. Then snaked this through on a regular basis --
every 6 months. Not the way I would want to solve the problem, but it
worked. If you are dealing with some frozen area that you can't get
to, then you are sort of stuck with a complete defrost until you can
"snake" it clean. PITA.