One thing to try would be to unlink and re-link to Facebook and try posting again.
To do so, swipe the home screen of the app to the right. Tap on "account" and next to Facebook, choose "unlink". Then tap the red arrow and log back into your Facebook account. Once this has been done, try posting your card again.
If that doesn't work check your settings by going into your "settings" app on your device and then click on "privacy". Just find the Facebook option and make sure the Red Stamp access is "on".
Contact [email protected] if you continue to have trouble and we'd be happy to help.