When your Beslist feed is missing data and Beslist recommend to complement your products you can easily download the XML file and convert it to Excel so you can easily find the product ID's that are missing data. Below the steps you can follow to achieve this;
1. Download your XML feed using the download link in the Feed table.
2. Use an XML to Excel convertor like http://www.luxonsoftware.com/converter/xmltoexcel to convert the XML to XLS
3. Open Excel and select the column where is missing data to sort out which product ID has an empty value.
4. Complement that product ID with the requested data
5. Save your product and regenerate the feed