Retraction Jamming Problem!
I had problems with jamming PETG due to retraction.  My model had many retraction moves, such that the amount of filament used during a printing move was less than the retraction distance.  I found that several trips through the feed gear flattened the filament, which caused two problems.  


The flattened (or ovaled) filament had trouble fitting through the round holes and tubes, and
Since it was thinner, the filament was not gripped as tightly so there was less force pushing it.


I reduced my feed gear pressure and the problem stopped.

PETG is not as stiff as PLA.  The multiple trips through mangle flattened it.