Add to calendar function

I've created a series of 3 events (webinars platform), where people can sign-up to 1, 2 or all 3 events. When they get their confirmation email and click the 'add to calendar' function, it seems to be opening a new calendar, not adding the dates to an existing calendar. This only seems to be happening in Outlook (not integrate). Have also tried in Hotmail and Gmail emails and they seem to be working fine, as do single instance events - its just when you sign-up for a series there is an issue in outlook.  Has anyone else experienced this and how did you fix it?  Many thanks!