I dont fill mine up all of the way. I have a different one of course. But, I've had other gravity waterers and they can be finicky.
Is there a manufacturer that you can contact?
I have a similar gravity waterer that my husband just picked up for me. It has to be level to work the best. We have ours sitting on a piece of cement. The rubber seal on the cap must be in there good and the cap has to be on tight. There might be a stopper towards the bottom of the spout to...