There are 3 little air holes in the metal base bit of an Evod. Check they aren't blocked. Check that the coil is seated right into the base and screwed in properly. Check that nothing is blocking the inside of the centre sticking up post bit in the head assembly.
When you draw on an evod you are sucking air through the little holes in the base, up through the hole in the centre post, it flows around the coil and wicks up the centre post through the mouthpiece and into your mouth.
If you're having a hard time drawing on it then something is blocked in there somewhere.