There is an extremely interesting post and discussion here : http://www.nadinemuller.org.uk/blog/e-musings/
Basically, students don’t like the idea of posts to blogs or discussion forums being assessed. They feel they have to be careful how they write (because the work is assessed) in a way that doesn’t seem natural in the normal ‘informal’ setting of a blog or forum.
Being more explicit about what you are assessing might help.