Derek Thompson points to this chart:
The best explanation I've found for why the stimulus didn't work is this graph from the GAO analysis of the stimulus act. It shows pretty clearly that the 76 percent of stimulus spending through the first four months went to fill in the gaping holes in Medicaid and state budgets. In other words, the stimulus isn't acting like a pole vault lifting job creation above the baseline. It's been acting like a crutch to keep state budgets and payrolls from imploding.