If the problem is the internal memory, then you have to try to free internal memory. Normally videos are what more memory, followed by applications, especially some of them, and photos. If you have the possibility of adding a microSD card that you copy all videos and photos, it is a good choice. The other is to delete them. And then, focus on ending those apps you occupy and you can see in Settings> Applications> Sort by size. Facebook be one. Maybe install Facebook Lite could be a good choice.
If you still do not receive notifications WhatsApp. Do not forget one thing. Go to Settings> Applications> WhatsApp, and confirms that, for some reason, there is off Notifications box.