mo.php file keeps getting hacked

For about a week now, every morning I find that pomo/mo.php file has been modified. I have done all of the standard things: replaced all of the wordpress files with a fresh download, changed all of my passwords, all of the plugins are fully up to date, changed the keys. My website has always had wordfence and all-in-one wordpress security plugins installed. I can’t figure out how the hackers keep getting in. I need suggestions on where else to check.

Thanks,
Kirk