Hey Richard,
How are you coding the calendar token into your emails? If you made the same mistake I did, and coded the token as an actual URL link, it won't work. It has to be coded in as plain text, which to me was counterintuitive.
If you're only testing it in the preview pane, that may causing problems too. I haven't see that happen to me, but I've noticed certain things don't always work in preview mode, and you need to look at the live link/email.