NFC Nexus S, make readOnly Tag error

Question

I try to make a NFC-V, NDEF tag to be readOnly. But it doesn't work. I tried the formatReadOnly(...) method, but it doesn't work.

I also tried to test if the tag can be made read only with the method: canMakeReadOnly(). The answer is false.

So how can I protect my tags? Does the problem come from the Nexus or from the tag itself?

Any ideas?