Can you confirm that you don't have any filters set up? If there is a filter you will see something like what's circled in red in the image below. You can clear the filter by pressing the clear button.
If there is no filter and you still don't see all the data then maybe the rows were deleted by accident. You can recover the rows by looking at the "History" of the data. Look in the bottom right corner in the View Responses Tab for the History button. Click on it then go back in time. You can then copy and paste old data into the current response table.
If you still have issues then please share the form with me (email@example.com) as a co-author and I'll investigate.
Instructions on how to share a form : http://forums.adobe.com/docs/DOC-2462