Could you please confirm if you have just one inactivity monitor defined on your table ? In case if there are multiple, then only one with lowest order will take the precedence and will execute. May be this is what is happening.

Also you can refer to the below KB article:

https://support.servicenow.com/kb?id=kb_article_view&sysparm_article=KB0524319

I hope this help.

Please mark this helpful if this helps and Accept the solution if this solves your issue.

Regards,

Kamlesh