Sunshine, welcome to the forum. I would be concerned about the small vessel. The airlock indicates equal pressure and that hints at an air leak. There are a couple of things you can do: Use some plumber's tape on the threads of the vessel and then screw on the cap. Tear off some plastic wrap and fold it into a ribbon about 1" wide. Wrap this around the threads of the small vessel and screw on the cap. As for your stoppers coming out, make sure the inside of the neck of the gallon jug is dry and the stopper is dry. Liquid on either lowers the friction between the rubber and glass. Are your stoppers number 6 1/2? That is what I use in a gallon jug. Worst case, get some duct tape and form two strips about 1/2" wide, push the stopper into the jug and run the tape from the jug, over the stopper and back to the jug. I do this with my demijohns because I do not trust the stoppers. See photos.