something similar happened to me with a tank carto, where I beleive the tube collapsed. I filled as in Snowdragon's video, did a primer dry hit and all of a sudden a ton of juice came up the drip tube. I tried working a long pin thru the tube and hit an obstruction. I gently moved pin around and eventually got it thru the hole. When I tried to take a puff on it I felt it close up again. Like you I used a light behind it and couldn't see thru it. I am thinking that when I did the dry hit on it, I put too much suction on it and that collapsed something internally. I've not been able to get it fixed.