It is trivial to cause inflation, but central banks are either not allowed or ideologically opposed to effective means of putting money in circulation. Many central banks have printed enormous amounts of money and inflated the currency less than they would like to because they're buying second-hand bonds with the printed money. They've been really frustrated with the inefficiency of this, which I find amusing and slightly suspicious. Just printing money and giving it away doesn't necessarily cause short-term inflation. The way in which it is done is the key.