Have you rebooted the phone to see if that helps? Hold in power button until turn off slide comes up. Turn off. Wait a minute, then press power button again until Apple logo comes up.
What device do you have, and what devices are being compared against?
If you have a very old Apple device, such as the classic iPad or the iPhone 4S or older, some Wi-Fi networks which operate on the 5Ghz frequencies cannot be seen, because of he hardware in these old devices. In other cases, iPhones will show stronger Wi-Fi networks right away, and weaker networks will either not appear, or will appear if the "Wi-Fi Settings" section is left open and allowed to scan a few times.