You may have a tiny switch on the side of the card and that could be causing the problem. Switch it and see if that makes a difference. If there is no switch and you're using a Windows machine, there is a Registry fix.
Press your Windows key and R together then type into the Run box
regedit
and hit teh Enter key.
From the File menu, select Export and note the location of the file and/or folder of the Registry Backup you're making. This is crucial so please don't skip this step.
Then, navigate to this key:- HKEY_LOCAL_MACHINE->SYSTEM->CurrentControlSet->Control->StorageDevicePolicies.
If Write Protect is a number 1, then set the value to 0.
Exit teh Registry from the File menu and restart the system. That should do the...