* Lack of evidence: There is no definitive record of which plays Shakespeare actually wrote. The only evidence we have is the First Folio, a collection of Shakespeare's plays published in 1623. However, the First Folio does not include all of the plays that are traditionally attributed to Shakespeare, and it is possible that some of the plays attributed to Shakespeare were written by other authors.
* Collaborative writing: It was common for playwrights in Shakespeare's day to collaborate on plays. It is possible that some of the plays attributed to Shakespeare were actually written in collaboration with other playwrights.
* Changes to plays: Plays were often adapted and changed over time. It is possible that some of the plays attributed to Shakespeare were changed by other actors or playwrights after Shakespeare's death.
* Misattribution: It is also possible that some of the plays attributed to Shakespeare were actually written by other authors and mistakenly attributed to Shakespeare.
Despite these reasons for debate, most scholars believe that Shakespeare did write the majority of the plays attributed to him.