iCal doesn't correctly respond to invitations

I have three invitations in iCal that won't die. I accept or decline them, they temporarily go on my calendar, and then beocme pending transparent grey again.

Since this started happening I have received and acted upon other invitations, they correctly add to the calendar and then stop returning.

I am running Mavericks 10.9. Calendar version 7.0 (1835). Mail version 7.0 (1822).

Thank you