After spending a frustratingly long amount of time trying to get the dental floss under the center pin I gave up. and looked around for a o-ring. I found one on those cheapie keychain eyeglass screwdrivers that have a cap. I think it came with my kayfun clone kit. that little o-ring slipped right under the pin using a tweezers. it works great. Now everything I screw on fires.
If anyone is have firing problems use look for those tiny keychain screwdrivers and use the o-ring from that.
I left a comment about dental floss on someone's youtube video where he replaced it with a Danco #60 O-Ring. I did temporarily fix my MVP with Dental Floss, but the same thing happened after a while. The pin would get pressed down from me constantly switching tanks. I eventually went to Lowes and got that O-ring he mentioned. 10 pack was $2.49, but if you're lazy, don't have a Lowes/Home Depot near by, or they don't have them in stock, there's a seller on ebay that offers them for around 5-6 bucks after shipping.
The O-Ring did fix the problem, but now I can't tighten the tank all the way. Well you can, but the airflow becomes restricted, so you have to find the sweet spot when tightening. I think my MVP will be staying home from now on. I'll go with my iStick and an ego twist as my backup. I like to carry 2 in case there's an issue with one of them, and it allows me to switch flavors easily, but the kayfun/istick combo I just got yesterday is working so well it would be hard for me to switch off like before.
P.S. I've tried to cut the O-ring in half with an Xacto knife and a box cutter, but that ring is so strong that it just bounces back and laughs at me.