Lenovo Yoga tablet not automatically connecting to wifi

My Dad has a Lenovo Yoga 2 tablet running Android 4.4.2 He is encountering a problem whereby, if the tablet is switched off, it does not remember WiFi passwords and he has to input the password each time he restarts the tablet. This is true of both his home WiFi connection and other ones, so I assume the problem is with the tablet rather than the router. This is a new issue and he says that it just started happening without an obvious cause. I have the same tablet with the same version of Android. I have gone through the connection settings on mine and compared them to his, but nothing seems obviously different. However, I am no expert myself so am not really sure what I should be looking for. Forgetting the connection and reconnecting doesn't work. Any suggestions to fix this would be much appreciated!