We had the same problem, we just added a little cereal to thicken it up and use the rest of stage 1. We've mastered stage 2 now, and moving into stage 3. Serving size for 3 is a lot more, and there are so many more choices. It's thicker than 2, but not by much. It also seems to depend on the brand. We use Gerber and Beechnut because our Meijer doesn't offer all the variations in both brands.
Re: What's the difference? (Silly question)
The consistency is thicker, and they start to introduce mixed ingrediants.