Mail not including attachments

For the past year, about half of the time, when I include an attachment (usually a PDF or JPG under 4MB) to an email, the recipient says that there's nothing attached. How can I get attachments to attach? I've tried attaching by going to the File menu and clicking "Attach Files...", I've tried using the little paper clip icon in the tool bar, and I've dragged and dropped files on my email. I'm using Mail 16.0 with Ventura, on a 16" 2021 MacBook Pro.