Just a couple of quick things, you've probably already thought of these...
1) Presoak The film prior to pouring in the soup.
2) Are they using enough solution to cover the reels all the way? (My first instructor told all the beginning students to use the same amount of solution. I have several tanks by different manufacturers, all need different amount of soup.
3) Are they using plastic or stainless reels? I have heard that some people have buildup with plastic reels. I have not noticed this myself.

Hope you figure it out!
Brian