Hrmmm, ok I just checked and of course he sells both. If the bottom unscrews it is "rebuildable" (has replaceable guts). The rebuildables that he sells are a tad strange, I've never seen ones like it. On mine the head is replaceable, his is the whole stem and the head is attached to it and does not come off. So I don't know much about his. Again if it is rebuildable, I'd take the whole thing apart, put it back together making sure all the seals line up and whatnot. Fill it with water and lay it down again, turn upside down, shake, spin around, etc. and see if there are any leaks and where they are coming from. If it just a disposable, I'd take off the mouthpiece and make sure that the little white silicone cap is lined up and centered, reattach mouthpiece, and make sure the mouthpiece comes down over the little metal post and makes a contacting seal with the silicone cap.
I've never had an issue with any of mine dumping, and between my wife and I, we have had quite a few. I occasionally get one to leak out through the mouthpiece from condensation and then I just wipe it out or rinse it. I've also had a few outright break (the tube just suddenly breaks in half) which kind of sucks, but those were earlier versions, and I haven't had any of the recent ones do that.
Lemme know how you do, and what you find out. If it turns out you just got a crappy one, I wouldn't give up on them I'd just try one from another vendor.