There are many reason this could be but we always recommend the 3 main troubleshooting steps.
The first step is simply powering the handset off and back on again, in a lot of cases, this will resolve the issue at hand.
The next step is a manual roam, the way to do this is below
For an iPhone it's
Press Settings.
Press Mobile Data.
Press Network Selection.
Press the indicator next to "Automatic" to turn automatic network selection on or off.
If you turn off the function, press the required network.
For a Samsung it's
From the home screen swipe down to access settings.
Tap the settings icon.
Tap Connections.
Tap Mobile networks.
Tap Network operators.
Tap Select manually.
The next step is removing the sim card from the handset and trying the sim in another handset, this will ascertain if it is a sim card or a handset issue if the problem is still present.
Another tip that can help, is putting the handset on to flight mode, powering the handset down for a minute and then switching it back on and turning flight mode back off.
If these steps do not resolve your issue then please get in touch with us on 0333 202 5000 or [email protected] and a member of the team will assist you from there.