Symptoms

Documents that are arriving on Manual Review activity as empty or with no field values extracted are not used in Online Learning after the review ends. 

Cause

According to the documentation:

1. Online learning will start collecting documents as soon as it receives the first corrected document from a Manual Review Operator.
-For a Document skill, this will be the first document where the region of at least one field has been corrected.
 

2. After the first document is obtained, the following documents will be collected:

-All documents that have passed through manual review.
-Some documents that haven't passed through manual review (their share won't exceed 33% of all documents in the training set and the test set combined).

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This means that if no fields were recognized, the document is not added to the set since no changes were made compared to the recognized values (which are absent). The fields need to be recognized somehow for its recognized area to be modified. 
 

Resolution 
Make sure the following rules are followed for the sample to be used in Online Learning:

• The documents' fields are not empty on Manual Review as they arrive; 
• The fields' recognized area on Manual Review is modified;
• If the documents' fields are empty, try training the Document Set on a few samples before starting the Online Training to make sure the values are extracted on Manual Review; 
• If nothing helps - try to re-publish all the skills used, both Document and Process ones. After that, please disable the Online Learning -> Publish the skill -> Enable it back again. Once done, repeat the tests.