I don't know the life expectancy of butterflies are, but my butterflies have died twice in the last week. I have 13 piggies and 3 chickens and I had 4 butterflies.
Butterflies get 'used up' by interactions like milking (manually and by milkers) and massaging. Their life expectancies are much shorter than chickens. I try to have 6 butterflies in my house at a time and I have to replace probably 4 a week. I have yet to have to replace a chicken. You can extend their lives a bit by singing to them.
Your Butterfly Milker may have a faulty induction nozzle. These were recalled under an FDA order back in 2009. If you check the serial number against their website, you can determine if you are owed a replacement module.
Until then, I'd move the milker into the Community Gardens.