No worries!
Since the app is already installed and you can access it outside the kiosk, there are no issues with app installation. The possible reason why you are unable to access it while on kiosk would probably be:
When you close the app, the app isn’t getting closed. Instead, it is placed in the notification tray. Since it is not closed completely, you will have to access the icon directly from the notification tray. But, as the app icon is not displayed on the notification tray while on the kiosk mode, it is almost impractical to open the application.
The only workaround to your problem is to include the given application in the Start layout. Then, customize the Start layout once again to add the app. Further, re-configure the multi-app kiosk policy with the new Start layout XML file. After the policy is associated successfully, the Start layout of the device gets updated. And, you an have alternative method of opening the app from the Start menu.
I hope it helps!
Catherine George
Hexnode UEM