This problem may be fixed by running the pdm_k_reindex command from a prompt.
Whenever a knowledge document is created, it will build the indexes and add the necessary information to the ebr files.
Also, whenever pdm_k_reindex is executed, it will build the ebr files if it does not exists and if the .ebr files exists, the command will fill the gaps if any.
Whenever there are amy Knowledge Document Search issues we recommend the customers to run pdm_k_reindex command so that it will refresh the index information in the .ebr files used by the Search Engine.