How long have you been using the same wick? People say they last up to 2 weeks.
I have a dozen tanks though that I have different flavors in and typically use three different ones in the same day, and when I get tired of a flavor, I wash it out and put something else in. So it's hard for me to say how long any one wick is lasting for me. I have to go by the wicks, and yes I did have one that started turning my liquid dark and the wick was actually stuck to the coil and I couldn't even get it the .5 mil out to check it.
Don't pull to hard though, I actually pulled a good one all the way out once by accident, and couldn't figure out a good way to get it back in, so I had to toss that too.