What is it that causes Jet Skis to porpose?

[pimpin michael]

I have two identical Kawasaki jet skis and one runs great. The other makes for a brutal ride. Even in calm water the porposing is horrible. When there is one person on it, the porposing nearly stops, but when two people are on it, it gets ANOYYYYYING! This makes no sense to me because isn't more weight supposed to help a water craft stay in the water??? So i installed a new ride plate that cost me quite a bit. But it actually made the ride even worse! So im hoping that someone out there has found a cure for this and an answer to why it even happens.