Can we get some more information, please?
Are you saying that the plug was loose?
And if so, are you saying that you torqued them to spec, and it still was loose afterwards?
Is it possible you somehow crossthreaded the plug when you were installing it?
Please don't take that as an insult . . . it's been known to happen. For instance, when you get rushed, distracted, etc.
Any chance the wrong plug somehow got installed?
Did you by chance drown the spark plug threads in antiseize paste? If so, that will make your torque readings inaccurate, and could lead to buggering up the threads
Without seeing any pictures, it's possible the threads in the cylinder head are damaged. You may be able to restore them with a special thread chaser, which is readily available at your local parts store. If that doesn't work, you may need to install a helicoil, timesert, etc.