Wet spots forming in your candles is one of the biggest complaints I find among candle makers. Wet spots, are caused by shrinkage and air bubbles that occur as the wax is cooling inside the container. The wax pulls away from the glassware giving it this particular look. The good thing is that it does not affect the way the candle burns; it just doesn't look very appealing.
As a candle maker, I have had some troubles with this myself, so I decided to see what I could do to minimize the problem. First, I put my jars in the dishwasher. Give them a thorough cleaning to get any dust from the warehouse out. Just make sure when they are done; they are completely dry before pouring. Secondly, the room that you pour candles in should really be between about 70-72 degrees for best results. Lastly, the temperature of the wax needs to be just right. Candle wax suppliers have a listed pouring temp. To high or too low of wax can being poured into cooler containers cause those troubled areas. You can also preheat your jars if you want to. The preheated jar will allow the wax to solidify slower, so those air bubbles have time to escape.
These are just some of the things I've researched and tried that have helped me.