Purchased an Creality Hi and experienced with different full filament reels that the printer would give FR2851 “Retract issue, Filament Buffer failure". Please resolve and click “Retry” button”” errors repeatedly with whichever filament was in position 3. We returned the unit for a new one and now we’re seeing FR2849 “Retract issue, filament cannot be returned to CFS hub. Please resolve the issue and click the “Retry” button” error.
When the lever on buffer is pushed down, after every error that allows the print to continue correctly. Which suggests that the buffer needs to be redesigned or is defective out of the box somehow. Has anyone else run into this problem and solved it?
Make sure you didn’t do what I did when I first set up mine and plugged the buffer pfte tubes in backwards. The side with 1 hole should connect to the printer. The side with 4 holes should connect to the cfs (in any of the 4). It will work backwards, but throws this error all the time. I actually did this again once when I moved the printer and cfs and it took me forever to figure out what was wrong 
Thanks for the idea but we’d set up the PFTE tube to go from one of the 4 holes (bottom) to the CFS and the 1 hole ( on top) going to the extruder correctly. Still getting the error though 
Okay. Only other thing you might try is to shorten the tubes as much as possible and make sure they have smooth paths without kinks. My original cfs, I replaced the bottom spring (search youtube for cfs spring fix), but these newer units do not need that.