Why QE has not been stimulative

"The Fed's Quantitative Easing program—which so far has consisted of the purchase of some $2.3 trillion worth of Treasury, MBS, and Agency bonds, thereby more than tripling the size of the Fed's balance sheet—has been so gigantic as to have been inconceivable before it started, yet it has not done anything to stimulate economic growth or push up the inflation rate. The economy is still mired in the Weakest Recovery Ever, and inflation has dipped to its lowest point since the early 1960s. How to explain this?

Perhaps the Fed should have done more?

Or perhaps it's because QE was never designed to be stimulative in the first place."