When would a monarch butterflies wings not flutter?

Much like human fetuses, butterfly larvae can sometimes have genetic defects that can affect whether they successfully complete the transition to adult butterfly. They can also be affected by bacteria or viruses that can disrupt their transition within the chrysalis. I have hand raised many caterpillars to successful hatching and release, but sometimes there are a few which fail to complete the processs. While it is sad and disappointing, it is simply part of the natural process. Hope this helps.