when it is not the right time for the person to receive the information, as in, they have not learned the lessons to be able to understand them, er, how to explain...it has to do with Source and soul/incarnation contracts...OR:

once i waited hours for a reading at a shop...it was finally my turn and the fellow who was doing the readings came out, took one look at me and said he could not read me, i was blocked from him. my higher self had said NO to this man being able to look. i had a bad hit off the guy too...at first i took it personally (as if somehow i had something horrible he didn't want to see) but then i understood why he was unable to read me.

anyway that's my experience, i'm sure OP has more to add.