icloud can't open documents
I have about 2 dozen icloud documents across pages, keynote and numbers, these have been working well across an ipad2, a snow leopard Powerbook pro and a Lion macbook (both chrome). However 2 of the earlier keynote documents now appear with their cover pages (so look normal) within icloud.com but will not download. On the ipad2 they appear as black/grey keynote icons and do not respond to any action.
One document I really could do with accessing again. All others look fine, all be it there are a few I would not want to lose in the same way. Is this a bug, am I missing the obvious or have I done something wrong? I did look into the folder where the documents are stored on the Macbook but could see nothing useful.
Any help gratefully received