Images not sending through iMessage

Hi there! I've been experiencing a couple of issues with sending images through Messages, which I haven't encountered before. These issues only started this year and have been ongoing for a while now.

1. Images are not sending through iMessage

- Internet connection is fine, as all other iMessages are sending successfully

- This happens whether connected to wifi or just using phone data

- The error message I receive is simply 'Not delivered'

- It is not an issue with just the recipient, as it's occurring with other people as well

2. Unable to view past images sent via MMS

- Sending the image as a text message works, however, when I later scroll back through the chat history, the images no longer load

- I see 'Tap to Download' in place of the image, as well as the file name and file size

- Tapping it causes it to display 'Downloading', but the file doesn't actually download

- This affects messages received and messages sent

Any help appreciated!