From personal experience, I would suggest that the problem you're having is the same one I had at first. It sounds to me like your plaster mixture is too thin.

Try adding what you think is way too much powder relative to the water in a batch and see what kind of a difference that makes. It's hard to describe the proper consistency in words but I find that if you go by what is suggested by Bruce it's a thick milkshake, quite thick, sort of like pancake batter.

Also, one other thing that still leads to this happening to me is if there is "wet water" still in my molds from when I rinsed them before pouring in the plaster. Make sure you've done a good job of shaking off the water from the molds before pouring in the plaster.