First check all the o-rings are correctly positioned on the top and bottom of the glass, also check the o'ring/seal at the top that stops air getting in after you refit/close the top cap after filling.
If the seals/o-rings are good it is a wicking problem and you need to use a little more cotton for the wick.
when wicking twist the end of the cotton to a tight point and feed it through the coil. it must have some resistance when sliding through the coil - it should pull on the coil slightly but not enough that you have to use force to pull the wick through. When the cotton is fed through the coil cut it so each cotton 'leg' is the appropriate length. make sure the cotton you have twisted to a point is cut away and binned. Feel the cotton into the juice ports - use a pointy tool or the blade of a very small screwdriver if necessary and fill the juice ports. The cotton in the juice ports should not be packed in - if it appears too tight take the cotton out of the juice ports and thin it by combing it out with a pin/pointy tweezers or similar.
Wicking is the most challenging aspectr about building RTAs and nobody gets it right first time. Don't get frustrated as practically everyone goes through the same learning curve at first.
try to use the method of feeding the cotton through the coil fairly tightly and thinning the wicks for the juice ports if necessary. Don't be tempted to use thinner cotton through the coil as this method seldom works - it may seem ok at first but the centre of the cotton will burn out quickly.