It's very hard to find the cause without more information. It could be a range of issues or just one. If you put it too much FO that could be it, the amount of colour can effect burning, size of the container, it's not necessarily the wick problem. The only way to work it out is by doing tests - make a candle without scent or colour, then another with colour added, then another with both, it's the only way to isolate what is happening. It could also be that you have the wrong size wick for the container. Sorry I couldn't help more, but it's not all cut and dried without experimentation.
The only other thing is ring your supplier and tell them the issue you are having and see if they can help.