Reindexing of Product Flat Data in magento:

You have to truncate all the 'catalog_product_flat_i' table:
where 'i' can be 1,2,3,4,....

In my case there are 3 table so i fire this query to my phpmyadmin:
TRUNCATE `catalog_product_flat_1`;
TRUNCATE `catalog_product_flat_2`;
TRUNCATE `catalog_product_flat_3`;