Most of the time, none of the 2 ^ 32 nuncios possible in a particular block header template will give a hash header corresponding to the job validation goal. In fact, the probability that this do not to arrive is exactly 1 on the difficulty of the network. At the moment, it is 1/6 379 265 451 411.
When no nuncio succeeds, you have to change something in the header of the block and start again. The typical solution is to change the extranonce field in the coinbase transaction and recalculate the merkle root. See What's ExtraNonce?