Accepted invites not showing as accepted in iCal

I receive meeting invites to my work email and accept them. Some from a colleague using iCal, some from people using other calendars like Google. My work email is different to the email I use for iCloud, which is my personal email.

The meeting is added to iCal, but the little circle next to my email stays as a question mark, not a green tick. Infuriating as the people sending the invite think I have not accepted.

When I accept the ones from iCal in Safari, there it shows the green tick next to my name. But not in my Calendar!

When invites use my iCloud email and I accept it works perfectly.

This is really causing confusion and wasted time, so any help really appreciated!