I found with my Snugpak Traveller sleeping bag that it was best to LUBRICATE the zips BEFORE you go. Of oCourse I found this out the hard way...naturally!
The shell fabric easily catches in the zip truck / carriage thingie. In 2013 I totally destroyed my two-way main zip. It was only through the kind ministrations of a colleague who happened to have been a seamstress that we were able to make the bag functional for the remainder of my Camino. It is now a one-way zip bag... Oh well... The bag is made very well.
So, this year, I obtained a dry lubricant, intended for this purpose and liberally lubricated all zips on the sleeping bag. I ALSO LEARNED to not just grab and yank, but to gently zip or unzip while making sure you are not biting into the fabric.
I hope this helps.