I have a spreadsheet that I created on my Mac. I've dragged the file to the iCloud website and it uploads (apparently) okay. When I open Numbers on my iPad it shows the spreadsheet but with a generic preview. i.e. the standard Numbers icon. When I click it the progress wheel churns for a while then I get an error message saying "Spreadsheet couldn't be imported. An unknown error occured." Does anyone have any ideas what to try? I've been able to upload and access other spreadsheets since this one.