Then I suspect the driver for the wifi chip doesn't work correctly in Windows 8.1.
Where did you get the wifi driver? Does the manufacturer of your laptop support 8.1? You can tell if the drivers are supplied for your model laptop.
If no 8.1 drivers on the manufacturer's site, then it is not supported. To use wifi, your bext option is to buy a USB wifi dongle that specifically says it supports 8.1.