GRB, can you tell us what specific products you have tried and where you purchased them? Until I recently started reading through Ingredients in Meat Products, I had no idea that there were so many available – at least 4 different kinds of soy protein isolates and 3 different soy protein concentrates.
But, as you've discovered, they are not strictly interchangeable! In general, as Phil and Dan have already suggested, non-fat milk powder is generally a filler with the ability to bind to water, while generally the soy protein concentrates also contribute to binding the meat mix – not so much making it "denser" as binding the proteins so thoroughly that the texture can become rubbery and hard to bite through.
Phil, I'd beg your indulgence with this kind of "cut and paste" bookish stuff, but I know how tolerant you are of ignorant punters like me!
"Nothing exceeds like excess."
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