This is a recent (since last week of January) issue with the Excel Web App. Someone from Microsoft responded here:
Excel Web App causes .xlsx files to become un-editable on Windows Phone
"The issue here is with the file format we are saving in Excel Web App (EWA). The web app is adding a new part to the file (that it should not be adding) and the phone app doesn't support this part so it then disallows editing. We should have a fix out soon. In the meantime, there is a workaround that you can use to allow you to edit the file on your phone. The workaround is to use the Excel desktop application if you have that available to you.
The workaround is to:
1. Open the file in the Excel application from your computer
2. Edit at least one cell and do a Save
You will then be able to edit this file on your phone.
I'll update this thread when the fix has been deployed to our service. I can't say precisely when that will be as their are various factors but we see this as an important item to address soon to unblock you. Until then you can use the workaround. Sorry for the inconvenience this is causing you. Thank You."
I had to get it to work by copying and pasting all the data into a new spreadsheet, which I then only edit with the phone (opening it in the Excel Web App locks the file again). Even copying it to my phone from Skydrive did not resolve the problem.
"Hi - Rob Long from Excel Web App team. We hear you and we're working on getting the fix shipped as soon as we can. But at this point the date is still not certain. I can say it will be out no later than end of Feb, and earlier if we can. I hope to get it earlier than that. I'll reply to this thread with more info as I know it, and again we're sorry for the inconvenience this is causing. Thanks"
Hope they get the fix out soon.