Some brands of filament spools (e.g. Renkforce 500g)have grooves and stuff like that where one might run into problems with the way vertex delta unrolls the filament.
(for example: https://www.conrad.de/de/filament-renkforce-pla-compound-175-mm-bronze-500-g-1528342.html)
I made this thing:
Which should help tu unroll the filament gently from the spool.