Simply put, monetary policy is far less effective in affecting real (or even nominal) economic activity than investors seem to believe. The main effect of a change in the monetary base is to change monetary velocity and short term interest rates. Once short term interest rates drop to zero, further expansions in base money simply induce a proportional collapse in velocity.
I should emphasize that the Federal Reserve does have an essential role in providing liquidity during periods of crisis, such as bank runs, when people are rapidly converting bank deposits into currency. Undoubtedly, we would have preferred the Fed to have provided that liquidity in recent years through open market operations using Treasury securities, rather than outright purchases of the debt securities of insolvent financial institutions, which the public is now on the hook to make whole. The Fed should not be in the insolvency bailout game. Outside of open market operations using Treasuries, Fed loans during a crisis should be exactly that, loans - and preferably following Bagehot's Rule ("lend freely but at a high rate of interest"). Moreover, those loans must be senior to any obligation to bank bondholders - the public's claim should precede private claims. In any event, when liquidity constraints are truly binding, the Fed has an essential function in the economy.
At present, however, the governors of the Fed are creating massive distortions in the financial markets with little hope of improving real economic growth or employment. There is no question that the Fed has the ability to affect the supply of base money, and can affect the level of long-term interest rates given a sufficient volume of intervention. The real issue is that neither of these factors are currently imposing a binding constraint on economic growth, so there is no benefit in relaxing them further. The Fed is pushing on a string.