What's wrong with this sentence: "Vous maigrissez." I think there's supposed to be something before and after maigrissez.
If you are working with the verb: "maigrir" to slim; to lose weight, there is nothing wrong with "vous maigrissez" if you are saying "you are losing weight, you do lose weight, you lose weight." Perhaps this is not the context, or the way you wish to use it? Please provide the English or the rest of the sentence.
Sra (aka Mme)