Quantitative easing "works" to a limited degree by easing liquidity through expanding the money supply (monetizing debt). It has the aim of:
"when the Fed buys these financial instruments, the money supply in the economy increases. This is evident when one looks at the U.S. monetary base, which shows that total amount of currency that is in circulation with the public, or held as commercial bank deposits in central bank reserves. (See Figure 3) [Forbes]
Quantitative Easing Cannot Substitute for Fiscal Policy
The Problem is that quantitative easing cannot substitute for fiscal policy. Ultimately the best way to inject money into the system is for the Federal Government to spend it directly on Goods and services. We've known this since Alfred Maynard Keynes wrote his seminal work on the subject. In our current system this is constrained by the need to "balance" budgets by issuing treasury debt to cover the money created.
Quantitative Easing (QE) sometimes known as "Helicopter" helps as "monetary policy" in the absence of fiscal policy by driving down the cost of loans. But it is limited in it's power during recessions by the creditworthiness of borrowers -- which is destroyed by the depredations of banking and the business cycle:
"However, commercial banks did not give all of this money out as loans. In fact, they kept a large proportion of this money created as excess reserves with the Fed. (See Figure 4)" [Forbes]
Usurious Debt Money is Unstable
This is also a problem in almost all countries that have imitated British Central Bank Concepts and let the banking system privatize on money. The banks create money through creating debt, but that does nothing to net stimulate an economy, because it only "works" as long as debtors have credit worthiness and the interest on the loans pretty much destroys that over the business cycle.
Thus, this is a problem not only here in the USA but in European Union. As my friends in the economics community note, the European Central Bank (ECB) will not allow any of the EU governments to create and distribute "helicopter money", so if you lobby your local EU government to do so, all they will say is "we can't - our hands are tied by the ECB.
Denying Necessary power and authority to local Government
One reason for BREXIT and the current mess is that the ECB ties the hands of local Government, in a way analogous to what the Federal Reserve does to State Governments. It takes a congress or a parliament, but both have "pretend" artificial constraints on actual effective "helicopter money!"
Centralization without Subsidiarity
The situation of EU member states is analogous to that of municipalities and states in the United States system. In both cases, centralization denies sovereignty to subsidiary Governments so they can't exercise basic functions of finance without either borrowing money at interest or taxing their people. This reduces their power and enhances the power of the sovereign banks. Thus Central Banks modeled on that of London or Florence can usurp powers that ought to belong to sovereigns by using artificial constraints like gold or silver.