making soy candles with essential oils-WICK CHOICE?

I have been making soy candles with frangrance oils and am now making them with essential oils.
Can anyone help me with respect to the wick!
Most of them have performed beautifully, but some have poor burning performance.
IE: wax doesn't burn to the edge hardly at all and/or the flame becomes dimmer and dimmer and eventually burns it self out.
With some research have found that flash point/specific gravity can play a role in this.
Some information suggests that a high flashpoint/high specific gravity needs either a larger wick or a hotter burning wick.
The larger wick doesn't seem to make much of a difference and I am even more confused as the essential oils with the higher flashpoints
are not the ones that are burning funny.
I know I need to many many many test batches, but if there is anyone out there who can assist...HELP!