cart/tank maybe I am using the wrong terminology. The small plastic mouthpiece with the supply of juice that is plugged into the cone/atomizer.
Some eGo-c tanks come with the hole already punched, some don't. The ones that don't - we usually recommend pre-punching them, but if you didn't - chances are there is a 'chad' - like in the voting card chads - that is hanging off the inside of the tank or perhaps even on the spike itself. Pull the chad off the tank and check the spike.
I 'pre-punch' the hole using the end of a ball point pen (after taking out the ink cartridge) - the closer to the size of the spike, the better - if it's a tad small than the spike that's even better - it tends to 'hug' the spike when the tank is inserted - less possibility of a leak there.
Check all air passage - along the flat side of the tank and also the air holes by the battery connection. Don't block them with your fingers when vaping

If you're doing that, you aren't the first
Depending on your finances, I'd also recommend the Kanger T3 in the 1.8Ω coil. Good Prophets has that as the default ohms in theirs. Get one T3 and then get a 1.8 ohm replacement coil and one 2.2ohm replacement coil in case you have some light flavors.