I also used the CLR method by laying the rad on it's front with cap on. Fill it 2/3 of the way with CLR and slosh it around every 30 minutes or so for an afternoon. If it is calcium scale from using tap water, it will dissolve in a few hours.
If you can see the tubes inside, you can see which are plugged when you slosh it back and forth.
It is a cheap fix if it works!
Hope this helps.