HT5012 Why I can't update my application?

Can I know why I can't update my apps? Few days all ready. Please check for me.
Every times when I want to update, my iPad will show me "can't purchase on this time, please try agn later."
Wish can help... Thanks.


Try reset iPad
Hold down the Sleep/Wake button and the Home button at the same time until the Apple logo appears
Note: Data will not be affected.