Impossible to practice one form of a word.
There's only one word in my Italian vocabulary that's worse than "Pretty good". And it is "dalla" http://www.duolingo.com/word/it/da+il/Preposition Actually it's "overdue".
The problem is that "Practice weakest words" doesn't help, it never shows me any exercise with that word.
Even worse, if I go to that link listed above and choose to practice that individual word I don't get to practice "dalla" either. I tried that three times and there were 14 exercises, none of them with "dalla", only the forms that already were at full strength.
I can also relate to http://www.duolingo.com/comment/1250819, but it didn't bother me as long as I was able to eventually practice all the words. This is not the case apparently.