This is most likely because the IIS user doesn’t have access to the private key for your certificate. You can set this by following these steps…
- Start → Run → MMC
- File → Add/Remove Snapin
- Add the Certificates Snap In
- Select Computer Account, then hit next
- Select Local Computer (the default), then click Finish
- On the left panel from Console Root, navigate to
Certificates (Local Computer) → Personal → Certificates - Your certificate will most likely be here.
- Right click on your certificate → All Tasks → Manage Private Keys
- Set your private key settings here.