Assignment

We have a problem with some of our advanced upload assignments not showing any feedback. Others in the same course do not have this problem.
Here is what we have done.
Set up a course with 2 Groupings
Added Groups to each grouping
Set up 2 identical sets of assignments so that each grouping has it's own deadlines.
Used these successfully for almost a year.
Then we decided that we wanted to upload some feedback files for one of the groupings. Now all pupils from that grouping are unable to see any feedback from any assignment. We have deleted all of the feedback files, deleted and re typed feedback after saving and still not feedback visible to students. It all looks fine from the teachers view. SO for this grouping we are back to handing out printed feedback files and notes.
We have tried the same for the other grouping and the feedback files are uploading correctly and the normal feedback comments are also visible to students.

Has any one got any ideas?

Rick.