Saving a document in a doc-based app

In a doc-based app i’m making, i am using write to file and read from file to write and read a list from a document. When i save a document the first time it saves it normaly and every thing is fine. when i open it again and save it after making changes it goes through all the code in the write to file handler fine, but says ‘Could not save file’.

any ideas?

thanks,

gecko