It's basically just acting like a straw, just check everything is tightly in place and you aren't missing any o-rings or seals. Flooding of the area around the coil can cause this because there is too much liquid to be vapourized so when you inhale you suck up some liquid as well. A quick fix to this (if your clearomiser or whatever is put together well and not leaking) is to get a small bit of wick and fit it above the coil, most coils ship with 2 of these flavour wicks but more can be added or removed to suit your needs. Adding another one just closes the holes off a little tighter and makes sure all liquid is wicked onto the coil instead of just flooding in. Reverse can be done if you get no vapour or getting burn - remove a flavour wick