In order to be able to export the values of the necessary attributes correctly and generate the feed file successfully when Use Flat Catalog Product option is enabled, it is necessary to take the following steps:
1. Go to Catalog > Attributes > Manage Attributes, choose an attribute you wish to manage and set the options as shown on the screenshot below:
2. Set the scope to "Global" as shown on the screenshot below:
3. Run re-index in your Magento admin panel or using php shell/indexer.php --reindex command on SSH.