Webhooks suddenly stopped working?

The webhooks for my organisation were set up before I joined and we haven’t changed them at all but today they just… stopped working. They are supposed to be pretty simple: the update.php file runs a git pull everytime the repo is touched, so that when we commit a change to master on github, the files update on our apache server. we’ve used this same setup for years.
Now, the updates just aren’t going through. When I look on Github, it says they are delivered, but the changes are not showing up in the server files. And, as I say, we haven’t changed anything on the webhooks or server. I just don’t understand how these are suddenly failing??