the problem is classical Keynesian thinking forgets that someone has to pay for all that public sector spending. as a stimulus it works but the economy soon becomes addicted to the injection of extra money. we are in this predicament precisely because too many governments followed this policy over the long term and didnt worry about the consequences if the injection wasn't available.
But we are spending the money: the government is using quantative easing to pump money into the economy but it's been given to the banks (who are sitting on it). We don't need to generate more money, just use it better. A commentator said the other day that rather than making the banks more liquid, we could give £500 to every household in the UK and get them to spend it.
I'm not sure that's the answer, you'll get what happened in Japan and people sit on the money but perhaps some sort of voucher scheme: we just need to get people spending again.