Cannot sync custom tones to iPhone 5s.

Exactly the title. I've made a couple of custom ringtones and synced them to my phone. It worked fine then but after a while, I noticed that the tones were missing from my phone and the ringtone was switched back to the default tone. I couldn't find the custom tones anywhere. Then I plugged the phone to my Windows PC (as usual) to sync the tones back to the phone. I did all the necessary steps to sync them but nothing works. I unchecked and rechecked the tones in iTunes and I synced them a hundred times but the tones don't show up in my phone.

And yes, I've updated both my phone and iTunes and tried again but nothing works. So, I'm at wit's end end here.