If the phone registers the SD card being installed, and other things can work on the card, then it is the app and not the card that is the problem. You would need to try changing your settings in the app and make them not use the card. Then reboot the phone and change the settings in that app again to choose the card. Should work then.
If it still won't recognize the card, then you may have to uninstall that app, again reboot the phone, and then redownload and reinstall the app.
NOTE: This is for saving things to the SD card other than apps themselves. That is a whole other thing.