@kmanstudios Printer is only 1 month old probably no more that 20 prints total. I did notice some small bits of debris in the Bowden tube on extruder 1 and removed and cleaned it. Spools are new to me (around a month) but I take your point about warehouse time, though they printed fine when I first got them. The spool is still fairly full, but being conscious of the issue I did regularly loosen the spool during printing, enough to reduce friction when unspooling but not enough so it could tangle.
Where I might have gone wrong is using the same nozzle for PLA and ABS, when changing filament I've just been letting it feed through until the new filament is clearly running through the nozzle. Is using the same print core for PLA and ABS generally a bad idea?
Incidentally, after a failed print due to grinding, I can unload the filament fine, cut off the section from the grind onward and load it again and it will extrude fine every time during load, and start to print the first few layers fine.
I'll do some more reading on nozzle cleaning, thanks for the advice.