It is due to the following situations: 1, the citation source is not marked accurately: the paper cites a document, but it is not listed in the reference list, and the duplicate checking system will think that there is no citation.

2. Incorrect citation format: if the citation format does not meet the requirements of references, the format is wrong, punctuation marks are used improperly, etc. Then the duplicate checking system will also think that there is no reference.