There may be few cities which may not be available on weather app. Try other cities on your iPhone that may give you a hint whether the issue is on iPhone or on weather channel!
Weather App on iPhone
If you want to switch between Fahrenheit and Celsius, scroll to the bottom of the list and tap . The weather data used in the Weather app comes from The WeatherChannel. If you have issues getting accurate weather information, tap the icon in the lower-left corner to go directly to the weather source.
About the Weather app and icons on your iPhone and iPod touch ...
https://support.apple.com/en-in/HT207492