If the bits used to hide are in the order RGBA the data cannot be extracted. This seems like the most likely order for the channels to be used. For example in this image.
The text extract function doesn't seem to work. However, if you put the alpha layer bits first, then it does work. For example.
This seems like a less likely order for the bits to be used(ARGB). Since the alpha channel is probably going to be at the end it seems like that would be a better option, or perhaps another check box to append the alpha channel instead of prepending it to the other color channels.
If the bits used to hide are in the order RGBA the data cannot be extracted. This seems like the most likely order for the channels to be used. For example in this image.