First, in a benign environment like the period of the Great Moderation between the mid-1990s and the early 2000s, inflation expectations were mainly forward-looking. Slack had a strong and stable link with inflation. And shocks were temporary and not very large. Also, there was no discernible trend in inflation. In this environment, monetary authorities had no difficulties in keeping inflation expectations well-anchored.
In the current environment, however, inflation is low and more persistent. In a Phillips curve framework, this might reflect more backward-looking inflation expectations, more persistent negative shocks, and/or a flatter slope of the Phillips curve.