It may happen that weather does not work. Your watchface does not have access to the internet, only your mobile phone. This means, for continous weather on your watchface you need to stay connected with your phone via bluetooth. The watch will get the weather for approx. a day from the phone and if it does not get new data it will show null at some points. This means, that there is no data for the desired time available and this means, that your phone did not push new weather data to the watch, either because its not connected via bluetooth, it does not have internet or similar problems.
Additionally, the garmin app on your phone does need GPS and/or internet permission to load the weather data - make sure that you have given the garmin app those permissions.
There is a general help page from garmin about there weather widget here which may help if you have problems getting the weather widget and an original garmin watchface to show correct weather informations.
Weather is just as accurate as the data that garmin provides and as up-to-date this data is. There’s nothing that can be changed about this. If you want to find out more about which weather source garmin uses, check out following
Here’s the quote from the link that may be of most interest:
This information is collected from the closest official weather station to your starting point. For the most part, local airports will be the official weather stations.
To test if weather does actually work, simply try one of the original watchfaces made by garmin and/or the germin weather widget - if they don’t show your weather either, one of the points above is the reason and then of course my watchface can’t show the weather either.