If you use the new Android Lollipop 5.0 on your Samsung Galaxy S5, it may be that the text from new WhatsApp messages on the lock screen is hidden. Instead, you only see "WhatsApp - hidden content".
This is a new feature of Android Lollipop, which allows you to hide contents of new SMS and WhatsApp messages on the lock screen. Thus, it is not possible that someone can read the content in a new arrived message when the smartphone is locked. If you interfere with this feature and you want to have displayed the complete message on the lock screen again, then you have to do the following settings:
Open on your Samsung Galaxy S5 (Also works on other Android Lollipop smartphones) the app menu and then the settings. Tap here on "sounds and notifications". Now scroll all the way down to the section "notification" and tap there on "While locked". A window opens in which you can now select "Show All Content". Finish!
If you now receive a new sms or new WhatsApp message on your Samsung Galaxy S5, then the content of it is directly displayed on the lock screen.