Charles Dumas (Lombard Street Research) proposes a good explaination of why the Spanish bailout won’t work.

The Spanish bail-out was a typical example of half-measures. It seemed clever to insulate Spain a week ahead of the Greek elections, so that “Grexit” could be possible without immediate major contagion into Spain. But Spanish banks needed equity, not debt funding that would leave them as insolvent as before. So the provision of debt funding by the Eurozone required the Spanish government as intermediary, to turn the debt – now effectively national debt – into equity for its banks. So private holders of Spanish government debt have effectively been subordinated – and will be for each further tranche of aid that is needed in future. Thus contagion was worsened, and quickly spread to Italy, which can only too clearly not afford its share of the Spanish bail-out.