First check the last file copied and restored to the secondary. Run the following queries on the secondary node.

The following query will give you the last log backup file that was copied

SELECT * FROM [msdb].[dbo].[log_shipping_secondary]

Then check what was the last log backup file that was restored.

SELECT * FROM [msdb].[dbo].[log_shipping_secondary_databases]

Based on the name of the last file restored, see what should be the next file that should be restored. Make sure that you check the files on both primary and secondary. One of the files might not have got copied.

Read the following post for more details.

http://saveadba.blogspot.com/2011/12/sql-server-log-shipping-could-not-find.html

You might get a better response if you posted this question in the 'Administration' section or the 'high availability' section