Could be a plugin conflict. It could be a rather poor attempt at spam. It could be a server issue on your end—have you set up any filters on the address your contact form is sending mail to?
Most importantly, have you tried testing this yourself? Maybe the people sending you messages are getting their point across in their subject line and aren’t including a body.
There are other contact form plugins available. Have you tried any of those? If so, do you get similar results?