I don't know why it didn't work for everyone else, the shaker cup works for me. I made sure to twist the lid until I literally couldn't twist it anymore, and made sure BOTH of the flip up stoppers were down and sealed and it worked like a charm. I love that it's 20 oz, that means if I have a bigger bottle of water on hand, I don't have those few ounces left over, I can just pour the whole thing in to make a shake. They're a bit runnier this way, not as 'shake' consistency, but the extra fluid helps me feel fuller longer. I gave it four stars because I'm not sure that the little removable rack in it helps much. Maybe I'll try making a shake one day without it to see what difference it makes, if any.

It comes with the Starter Kits, so you may consider getting another shaker that works better. This one does not seal properly, so shaking is a mess.

This shaker came in my 4 week weight loss kit. I was so excited to get it, however I was very disappointed with it once I used it. The lid doesn't fit well and is not easy to use. It leaks horribly. I am using my water bottle instead.