Yea I know what you mean. I think it has to do with both. Sometimes I find I'm flat if I don't drink a shitload of water before bed and eat fewer or less voluminous carbs. Personally I enjoy the taste of dextrose added to my shake, though. The hydrolysates+leucine do not make for a very delicious combination.
On the other hand, I'm sure subbing dextrose for other carbs will get you better results. Before you know it, BAM, ya got abz.