Leonie,
This has happened to my viili, too. I thought it might have been because I was feeding it frozen milk for a while (the cow was dried off), but I found when I could get fresh non-homogenized milk, and later raw non-homogenized milk (from someone else's cows), it perked back up and doesn't make lumps anymore. If you get a batch you don't like as much, whether it's lumpy or it just goes a little too far in the fridge without getting drunk up, try making cheese:
http://flog.posterous.com/cheeses-0 (my fermentation log, or (f)log). If you do, let me know what your daughter thinks of it!
Bekah