Not that I've tried it this way, but it just occurred to me, could it be that you put the leucine in the container first and then the water?
I know someone who, till told to do differently, attempted to make protein shakes that way. It's a bad method; the protein powder sticks to the container, which is a problem you mentioned.
I don't know but perhaps the same might happen with leucine.
Other than that, I just can't imagine how to get your result.
To be very specific, what I do is add about 8 ounces hot (preferably, but doesn't have to be) water to a glass Mason jar or otherwise-obtained jar such as one that had held pasta sauce, then add the leucine, close the top, and shake for about 15 seconds.
Hmm, maybe the glass has something to do with it? As I have never tried plastic, I can't say whether leucine tends to stick to plastic or not.