Just like the OP, I had declined a single instance of a recurring meeting from an invite I have previously accepted.
Note: Any changes to the series on your calendar will be lost since you have to delete and re-add the series. ...but, that's kind of the point :-)
This just worked for me in Outlook 2016 since I still had the original invite:
- Confirm you have the original invite (Deleted Items, Inbox, etc.)
- This is important for restoring the series on your calendar after deleting it in the next step.
- Delete the series from your calendar
- Choose "No" when asked if you want to send a response to the originator so you aren't spamming them.
- Go back to the original invite and choose "Accept" without sending a response to the originator
- Check your calendar and all instances in the series will be there (even the one(s) you previously declined)