Raster to Vector Problem

I am having problems converting a simple gray scale line drawing to
vector. I thought it would be simple, compared to some, but it just
won't convert. Any reasons? Any suggestions? This is the first time a
scanned image came from a plastic like, translucent paper. I have
several of these I need to convert so if someone has a program which
works I would appreciate the help. I have uploaded the small and
complete pictures.