Switch your coil... you might have a dud that isn't wicking correctly.
But aside that, you have to ensure that you don't have a seal leak. Take tank off the mod, empty the tank, put in a new coil, put the tank together, close the airflow and suck on it.. you should not be able to suck in very much at all, if you do, you have an air leak somewhere, either bad seal or might actually have a faulty tank (though I've yet to see a bad Eleaf tank due to quality control).
Make sure your glass isn't cracked or damaged and sitting properly.
If sucking on it appears ok, just fill and suck on it a good 4-5 times (again, with the airflow closed) to get the priming going, but let it sit for a good 5-10 minutes, suck on it again a couple of time, put on mod, set wattage to about 25W, and fire it gently to make sure you aren't getting a dry hit. increase wattage to around 35W for a fairly standard setup.
If you aren't using Temp. control, consider it.. it is a fantastic way to vape regardless of what the haters say.
Make sure that you stay within the coils limits.
Melo tanks are great tanks (Melo2 is my go-go, my daughter uses both a melo 2 and a 3). They biggest if only flaw is opening them to switch the coil, they are difficult to open.
But the beauty of them is 1) there's dozens of brands of coils that you can use with them, 2) the coils are quite easy to rebuild 3) they are excellent for flavour, and heat with a proper coil as the vertical cartridge allows for a fairly huge coil. (I've been rebuilding mine, using NI for temp. control, 26G, 28G, and 30G, at 4mm size, and they've been excellent)