Install Norton Internet Security on up to 3 PCs - doesn't work

I received an email from Norton this week telling me that my purchase of Norton Internet Security allows me to install it on up to 3 PCs.

I currently have it installed on one PC only.

When I try to install it on a second PC it tells me that "Norton Internet Security has no remaining installs".

I've tried emailing customer support but they haven't responded.

Any suggestions?