Firstly, empirical evidence which accumulated in numerous countries increasingly undermined the Phillips curve theory as inflation accelerated but the hoped-for reduction in unemployment either failed to materialise or did not last for long. Secondly, economic theories were developed which explicitly incorporated learning behaviour.
Assuming adaptive expectations it could be shown that "the" Phillips curve (with the unemployment rate on the horizontal axis and inflation on the vertical) would shift upward over time. The true picture would then consist of a short-term Phillips curve where the trade-off holds for a while, but a long-term Phillips curve which is vertical.