Probably not all of the images were cached on the device. To insure that it will work offline, you need to install the app and then view every image. Once you have done that, turn your device into airplane mode and then confirm that the app will work.
Some professional features, like forms, do not work when the device is not connected to the internet.
There is a publish option, Wait For Images To Download, that will force all of the images to be loaded into the cache before the app starts. This will insure that the app will work offline. This will cause a delay when the app is first opened up on the device as the cache is being loaded, but afterwards, it will be quick as normal.
NOTE: If you clear your web application cache on your device, the images will no longer be stored on your device and you will need to repeat the viewing of the app and all images in order for the app to work offline again.