Redmine

We came across a problem during the using of Redmine : mail notification can't be triggered in two Issues of one Project, but we got mail notification before, and there is no similiar situation in other Issues. We want to know, is this a bug in Redmine? BTW, the fuction went normal after we copy the Issues into a new one, but the original Issues still got this problem. Please help to check it,thanks!

Environment:
Redmine version 3.3.2.stable
Ruby version 2.3.2-p217 (2016-11-15) [x86_64-linux]
Rails version 4.2.7.1
Environment production
Database adapter Mysql2
SCM:
Subversion 1.7.14
Git 1.8.3.1
Filesystem
Redmine plugins:
_query 0.0.2
clipboard_image_paste 1.10
easy_wbs 2016-0.0
redmine_agile 1.4.2
redmine_ckeditor 1.1.4
redmine_contacts 4.0.6
redmine_custom_workflows 0.1.6
redmine_dashboard 2.7.1
redmine_dmsf 1.5.8
redmine_favorite_projects 2.0.3
redmine_knowledgebase 3.2.1