Hey, I bought those XTank 4 coils too but got my Melo 3 tank just today. Obviously tried that coil and it's not firing - "Check atomizer". Checked the coil with multimeter and got 0.6 Ohm readings so the coil is good.
The only culprit I can come up with would be that coil is somewhat short and not touching the bottom of the Melo base part.
Grandmato, any suggestions?
Have you tried the Melo 3 tank on another mod or two ? Sounds like you've most likely have a problem with the tank's 510 connection pin not making contact with the mod's 510 pin. I'd try other mods first to see if it works on them. You may need to gently, using fingernails or a pin to pull the pin out a bit. Gentle being the key word here