Calendar appointment sync between devices failing.

Hi, hope someone can help me. I rely fairly heavily on my calendars for my medical appointment reminders but I found today that there is no sync between my iPad (which is where I mostly put them in) and my phone. This has never been an issue until the last couple of days. It was working well at least until Monday afternoon which was the last time I had a successful sync. An appointment I put in yesterday did not propagate to the phone. Nothing has changed. OS updates are all done (whichever the last one was) and the network hasnt changed. I'm at a bit of a loss