The first suggestion I have is that you make sure you have a stable connection as this is usually the problem with the WhatsApp app and it can even cause problems such as not being able to exist a group at all. A strong signal is of little use if the quality of that signal is poor.
Also, try and make sure that you have saved the contacts onto your mobile.
Should it be none of these 2 that are the problem, visit the www.whatsapp.com and go to the Groups FAQ which may help you further.
Beyond this, you may need to reinstall WhatsApp after backing up your chats.
Hope this helps.