I tested my Protank on my brand new MVP (V2) last night. It wouldn't fire. I GENTLY pried the center post out a little on the Protank (and centered it better -> reduce chance of shorting) with a jeweler type screwdriver and it fired fine.
I have to pull out the center posts on my T3 heads to get them to fire on my eVic sometimes. I just use hemostats for that.
I wish the center post lengths were standardized better, but it's an easy fix.
Just don't pull the center post on your clearo out too far

It should only take a fraction of a millimeter.