Rather a lot of people have these problems with ProTanks.
First, quit staring at charts and stop getting lost in the shoulds. A Kanger coil works at it's correct voltage, not what "the chart" says it should work at. If you can get past, OMG, a 2.5 ohm coil actually working at a feeble 3.4 volts there is hope for you. If you can't accept that possibility keep burning crud onto Kanger coils starting them at 4+ volts, ruining them in under two minutes, and move on to another device.
I won't even use 1.8 ohm coils because

even 3.2 volts can be too much for a new 1.8 ohm coil. There is no room to adjust voltage down with the 1.8 ohm coils. Always start a Kanger coil at your lowest available voltage or wattage and only move up as needed to get the vape you are looking for. Don't go by "the chart" or feel sexually inadequate if you manage to get a 2.2 ohm Kanger coil to produce tons of vapor and taste right at a whopping... 3.2 volts. I won't tell anyone.
For the people who still insist that every device is a straight line thing that must conform to a theoretical volts/watts chart you are gonna be disappointed with a lot of devices that refuse to comply. You are also ignoring the non electronic aspects of the design that don't care about electronic theory. Once you have trashed a Kanger coil by starting it at too high a voltage/wattage and burned crud onto the coil the deed has been done - it's gonna taste like crap until you replace the coil or clean it. Once you have burned flavoring components onto the coil your vape is a mixture of burning crud and liquid vapor. You can't reverse the process by dropping the voltage
after you do that.
Keep the volts/watts low on these things when you start and only adjust up as needed. Believe it or not - that can work.