I have this problem both in moodle 2.5 and 2.6 on my local installations of Moodle.
I create one feedback instance on my site's front page and I "map" it to all courses. I add the feedback block to the courses where I want that feedback instance to be available.
Then I log into one of the courses as student. I click on the feedback name in the feedback block. The front page feedback is displayed, well only it's title, but ... there is no link to Answer the questions...
What am I missing?
If I click the Preview icon, I get the message "Sorry, this activity is currently hidden".