If you have than 10,000 records it will export the 1st 10,000 records only, where as the rest wo't be exported.
So just a change can make a great difference .
Follow the following steps
1. Log-in to the SQL Server where the
2. Open the "OrganizationBase" table
3. Find the Column: "MaxRecordsForExportToExcel" (it will be towards last)
4. Change the value from 10,000 to the desired value
5. Commit the change to the database
6. If required start SQL & IIS also.
After that you will get it done.