Welcome to the forum Davidwh.
The simple answer is that with use, the coil gets gunked up from the juice it burns,
this is turn flows back down the wick and into the juice, so in effect it works like a big
circle. The easiest thing to do is fit a new head, but there are some steps that you can take
with the old head, although it doesnt guarantee to cure the problem, and it will void your warranty.
Firstly remove the top of the tank and then remove the rubber cap that fits over the head.
Now you have full view if the coil. Burn the coil in quick blasts of 3 or 4 seconds, until you
see it start to glow. (its best to do this with an empty tank) Once it starts to glow you know that
the gunk has been burnt away. To take it a step further you could also remove the head and wash
it in boiling water, this will get the gunk out of the wicks too.
If you dont fancy doing the dry burn, you could just take the head off and pop the whole lot in boiling
water for a few minutes.
Put it to one side and allow the wicks to dry out, then put it all back together, dont forget to
pop the little rubber top bit back on.
Just a word of warning, any type of dry burning does run the risk of the popping the coil,
so do make sure you have a spare at hand.