You know what? I have not shaken my bottle of nic once at all to be completely honest, could it really make that much of a difference?
This is the main source of your problem, especially at 100% VG Nic.
One possibility is that your nic has oxidized. There are three things you want to keep in mind with your nic. Exposure to UV, exposure to oxygen and exposure to heat.
Oxygen degrades nicotine. Every time you take the cap off your nic, you expose it to oxygen. One thing you could do, is store a smaller portion of your large bottle of nic, into a smaller bottle. This way you are not exposing oxygen to your large bottle every time you make a mix.
I used 100% VG nic for about a half a year. The vigorous shaking required to get out the hot spots every time before a mix was a pain in the butt. I've switched to a 50/50 VG/PG blend and I don't think I'll ever go back.