Where's the location for relay? I may have this same issue.its possible glow plug relay issue. other friend of mine with MK6 Jetta has exact same issue, replaced the relay, she fires up quickly.
last weekend, I did a swap of glow plug relay from 2015 Passat to 2013 Jetta TDI, noticed she cranks and fires up normally then swap back to the old one, she does it again randomly. have to keep testing it to see if the pattern is the same then replace it. I'm watching her car for a while.