I can not enable iCloud Backup....but i could in the past...

when i first installed iOS5 on my iphone 4, i was able to use icloud, but i didnt have enough space, so the backup did not complete. Now, 3 days later, I purchased more space on iCloud, and when i try to turn on iCloud, i get a window that says " iCloud Backup Failed- There was a problem enabling iCloud Backup". Anyone have any ideas?? thanks....