Please check the signal in your area by following this link https://giffgaff.com/coverage-and-service
for the network status or coverage. The network status should give you details of any local issues, including planned work, as well as showing signal strength & outages.
Please remember that the website is updated hourly and may not display any issues at the current moment. If the signal looks all good then you can perform a manual roam Please check out this http://community.giffgaff.com/t5/Using-giffgaff/Force-the-change-of-SMS-settings/ta-p/4871794
guide on how to perform a manual roam.
Also please turn off your mobile device off for a couple of minutes and on again to see if that helps.
If the services look good i suggest you replace the SIM to see if that helps.
You can order another free SIM from your spread page here https://giffgaff.com/orders/mgm or you can can pick one up for high street retailers e.g. Tesco, Iceland and Poundland for around 75p
All new giffgaff SIMs are 'trio' SIMs made up of a Standard, Micro and Nano. If you wish to form the Nano or Micro size SIM all you will need to do is push out the middle of the SIM as shown in the image below:
When the SIM arrives perform a SIM Swap. The SIM swap will transfer your current number and any active goodybag / credit over to the new SIM and can be performed any time between 4.30am and 9.30pm
You can follow this https://community.giffgaff.com/t5/Using-giffgaff/What-is-SIM-Swap/ta-p/7320084
fantastic guide i found on how to perform the SIM Swap or you can follow the SIM Swap video tutorial below: