Help Istick locking watts

[Phoenix .]

My istik takes only 3 hrs from empty to fully charged, then I only get about 2hrs use before it locks again, can't understand why this is happening. I don't change the settings as I use the same evod/iclear atties and heavy VG juices.

I have put my blue away not using it until I get a valid answer as to why this is happening, is has been said that when fully charged that is when the locking happens, I just don't get that, you should be able to fully charge it or what's the point?

I have checked everything, wall charger is new as is usb, raised centre pin, cleaned all attachments before use.....used to work fine when I first got it, so what has gone wrong?
Any more info /help ?