Strange spots of no printing?

Hiya, I use the local library's Ultimaker 2+ twice a week. I'm trying to print these glasses for cosplay but for some reason I keep having this issue where the printer won't print in some areas. It's clearly going along the track it's supposed to but nothing is coming out in these spots and then it almost fades back in?? I've knocked the speed down to 60%, extrusion to 105% and added 5c to the temperature but still nothing :/