Do you mean login to the WiFi network and not login to the tablet?
If you mean the WiFi network, go to Settings > Wireless and Networks > WiFi, select the appropriate network and then select Forget this network.
Disable the WiFi and the re-enable it and let it scan for new networks. Select the wanted network and insert the password. Connect to the network and then prove it works OK by using an app etc.
Restart the tablet and check if you have to reinsert the password.
If it still fails to retain the password I’m not sure with your tablet how to access the advanced WiFi settings (if any). Select the wanted network and then perhaps press the right hand soft key at the bottom of the screen to open another menu box. If there is an auto reconnect or save option, select it.