There are a number of reasons why some callbacks may fail; it could be a temporary network issue. But Paypal seems to be quite sensitive to non-western characters or other non-alphabetical characters in orders. This seems to affect the success of callbacks.
To address this, try the following:
- Log into PayPal
- Click the 'Profile' link in the menu bar under 'My Account'
- Click the 'Language Encoding' link under the 'Selling Preferences' column
- Click 'More Options'
- Set 'Encoding' to 'UTF-8'
- Set 'Do you want to use the same encoding for data sent from paypal to you?' to 'Yes'
- Click 'Save'
UTF-8 is a larger character set, so using this setting should better handle orders with accented characters, ampersands and so on in the order.