Not getting enough protein
Ok - now this may not work for you, but it did for me. If you are a peanut butter fan - try adding peanut butter to your protein shake. I would add a couple tablespoons of PB2 powdered peanut butter to my shake and that would make it more tolerable to me. It would make them less sweet and more tasty. The shake plus the pb2 added enough protein. Not saying I loved it - but it made it more tolerable! I've seen other people who added in powdered milk to things for additional protein!
It may just work for you! Get your protein in!