OK, lots of people have had issues with the Tradfri if they do not hold it until they see the light start to fade and then continue to hold it for a few more seconds. It’s a long time. 
To factory reset the TRADFRI control outlet, press and hold the reset button (pinhole underneath the light, located at the top (bottom if UK) of the outlet) with a paperclip until the white light starts fading. Hold onto the button for a few more seconds, then release. After this, the outlet will automatically connect.
Here’s a discussion thread.