Hi @Glenn Hackett ,
Thank you for posting in this community.
1.Please check if "send mail" action does not work in both SharePoint workflow 2010 platform and SharePoint 2013 platform.
2.Please add the actions "log to history" before and after action "send mail" to check if the send mail action is performed.
3.And you can also add "pause for a duration" before the "send mail" action, see if it could help.
4.You may also check if there are some related ULS log.
Also, I agree with Andrew Geddes that this might be a good opportunity for you to use Power Automate.
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