Time Machine Not Backing Up to WD My Passport for Mac

I have been using a WD My Passport for Mac to backup my Macbook Pro (Retina, 13-inch, Early 2015) since 2015. Earlier this year it just stopped backing up. I am running macOS Mojave Version 10.14.6. The strange thing is that there is no clear reason the backups aren't happening. The disk is visible as soon as plugged in and I can access all the old backups via Time Machine. There are also no error messages when hitting "Back Up Now". It will simply show as "Preparing Backup", as per the usual, but eventually the "Preparing Backup" message will disappear and nothing will have happened. It will then show the time for the next backup (usually an hour or so later), also as per the usual. Once this time comes around the "Preparing Backup" will show again, nothing will happen, and the cycle continues.

Does anyone know what the issue could be? Given I can access the old backups it doesn't seem like the disk has been corrupted so I am hesitant on dropping the money on a new hard drive but at the same time really need to run a backup as soon as possible.

Worth noting, in case it is relevant, that the hard drive is full and has been for the last couple years but old back ups had been being deleted to allow space for new backups up until it stopped working.