I agree a weak signal is my most common issue. Also check you have the wifi password is correct.
Are you really using WEP? Have you used option 1 "Scan for wifi..." this will find your network and most times automatically select the encryption type correctly.
Also I have had an issue where I restored to factory firmware or install another firmware from the repo and the network never works. If you reboot via the serial console:
Check and see if the Warp Core wifi device driver is loading, if it isn't then this will be your problem.
Copy the new firmware to /tmp and run the command:
sysupgrade -n <firmware file>
The "-n" blows away your confirguration which is not what the factory reset seems to do reliably. I've fixed 6 separate Omega2+ using htis method.
Now reboot while connected to the serial console and see if the warp core wifi driver banner is shown.