Hi lollystewart, please ensure you are using at least our 1.3.6 Plugin update, also please see our Documentation for large data exports as your exports may be exhausting all allocated memory within WordPress resulting in a 500 Internal Server Error (blank page), for Order exports of more than 750 Orders at a time you will need more than 256MB of memory.
http://www.visser.com.au/documentation/store-exporter-deluxe/usage/
