Do you know how to use a gravity bong? You slowly lift it up out of the water to create suction. The suction will draw the smoke into the chamber. If you didn't do that, then that's your problem. Otherwise, it could be that it's not air-tight (aka, you sealed it badly).

I am not exactly sure how you could be having problems, though. Me and a friend made a gravity bong using an apple as a bowl a couple years ago, and it worked perfectly, and even gave the smoke an apple taste.

As for the waterfall bong, I'm pretty sure you're being sarcastic, Coelho :-). If you aren't, then you probably made the hole at the bottom too big, and thus there wasn't enough suction to draw the smoke in.