S5, nozzle 2, PLA, not printing

[StephenRobertson 0]

We have an S5 that’s been having trouble with nozzle 2, PLA.  Here’s the lead up. The printer would indicate out of material with plenty left on the spool, turned out to be a clogged nozzle  (flow sensor, the little wheel with a Hall effect sensor I assume) not seeing any motion, so thinks the material is out, I assume this is how it works. (Is there a sensor to see that distance between the drive and idler wheel, i.e. if the gap is too  small the material is not there?). 
We cleaned the nozzle and it was fixed for awhile. Then ran into a similar issue but it seemed that the nozzle just wasn’t getting hot enough for the material to extrude, so we replaced the whole nozzle assembly and we were up and running again.  Now we’re down again with the following issue:  the material loads and extrudes fine on material change, but when it comes time to print nothing comes out, the feed motor is moving, the filament is moving, but it’s as if the filament has been retracted too far just before printing, leaving a big gap before the barrel.  Help!

Edited May 8 by StephenRobertson

[Gero 7]

Are you using cura4.9? There seems to be a bug which causes all the Ultimaker S-Line printers not to correctly purge/re-retract at the start of a print with the second nozzle. Switch to cura4.8 and try the same print again. The "Purge blob" function also doesn't work in cura4.9, so for Ultimaker printers cura4.8 seems better for the moment.

[StephenRobertson 0]

Ok!  Thanks!  I’ll revert and try it again. Should I roll back the firmware too?

[StephenRobertson 0]

That was it, back to printing now.  Thanks so much Gero. (That’s was a fairly obvious change in condition that I missed, the cura upgrade, live and learn). 

Edited May 8 by StephenRobertson