Can you upload an example? Are you on version 8? I am not sure why you wouldn't see all your worksheets, unless you've hidden them once they've been placed on a dashboard. By default, that shouldn't happen, I don't think. Unless I am missing something.
Are your worksheets hidden after they've been applied to a dashboard? If so, go to the sheet and unhide them. Then, the worksheets will show up on the dashboards as options.
Hope this helps!