Hi, Sylvia,

ServiceNow is using a watermark in the email body to match with the existing record to do an update for inbound reply emails.
Please check first to see if any watermark in your emails.
Also, another point is to make sure your email has prefixed with Reply email prefix in the subject line.
You can check the inbound email processing workflow from below wiki page.
Inbound Email Actions - ServiceNow Wiki

Mark it as correct or helpful if this helps

Thanks
Veer