OK, this may sound really dumb; but can't deflation be solved instantly by just printing more money?
There is a very simple answer to this question: people and banks can choose to save the money printed instead of spending it on goods or investment projects, so prices don't increase anywhere due to significant demand shifts. No need to write 800-900 words about it.
If the secular stagnation hypothesis is true, though, then deflation isn't even the biggest problem anymore. The culprits are the usual ones: aging workforce, labour market adjustment failures, etc. The consequence is that banks don't see any projects as profitable enough (the expected interest rate on these loans outpace inflation, so the real interest rate is negative).
This. The money has to actually come off the bank balance sheets and move around in the economy purchasing goods and services to have any impact. And as someone already mentioned, once prices have begun to fall, it is very hard to make that happen.
People aren't going to go out and buy a new car that they know will be cheaper a month from now. But they're scared and will keep their money in banks and fixed income assets even when interest rates get so low that they're paying for the privilege of doing so. (That's what's happening in Europe right now.)
That is one of those situations where governments have to use the fiscal policy route and actually start spending some money of their own accord. Monetary policy by itself isn't going to be enough - it's too late for that at this point.
