The equation of exchange says money multiplied by velocity equals price level multiplied by real output. It is true by construction, since velocity is defined as the residual. It only has predictive content if velocity is stable, which it is not.
Velocity falls when money is held rather than spent, which is what happens when precautionary saving rises or when banks park reserves rather than lend. That is why enormous increases in the money-supply during quantitative-easing produced no immediate inflation.
Example: nominal GDP is $29 trillion and M2 is $21 trillion, so velocity is 1.38. If M2 rises 10% to $23.1 trillion and velocity falls to 1.26, nominal GDP is unchanged at $29 trillion.
Related: money-supply, quantitative-easing, real-gdp, gdp-deflator, inflation-expectations