In the free trial version, you can only preview a part of messages. Full messages can be viewed in registered version. Besides, it also doesn't support pictures preview for free. Thus, if you would like to use it without limit, you'd better purchase the WhatsApp Transfer for iOS.
If it still doesn't enable you to preview full message after registering after registering, try to relaunch the program.
Feel free to contact us (firstname.lastname@example.org) if there is any other problems.