I can assure you that this issue is actually more common than you think.
Solutions for this issue:
I generally find the Xcode's settings and by signing out of my account and then signing back in again resolves my most of those issues.
You can also try the older mac that already used up the one allotted development certificate. Here you must export the developer profile from that machine and that will solve your issue.
You can also double check the build settings in the project and make sure that it is looking for the right certificate. It is common for the settings to make decisions on their own.