It might be because of pagination. Check you /wp-admin settings.
Also, try to set numberposts=-1.
Source:
https://github.com/picandocodigo/List-Category-Posts/wiki/More-parameters-you-can-use
numberposts – Number of posts to return. Set to 0 to use the max number of posts per page. Set to -1 to remove the limit. Ex: [catlist name=mycategory numberposts=10] You can set the default number of posts globally on the options page on your Dashboard in Settings / List Category Posts.