Calendar entry won't delete in Mac OS

Since OS 15 on my MacBook, I have Calendar entries that won't delete. On my Calendar I have multiple calendars – most are iCloud and I enter those manually, which are fine, but I have one calendar that brings in meeting appointments that come through my university email. There have been a number (some repeating) that show as "Imported Read-only", and those I cannot now delete. I previously could with no problem, but now they stay even though there are Cut, Delete options in the menu. They are very annoying, as they prevent me from using the Calendar as a record of what I actually do.