Unlike the U.S. and the U.K., where inflation has started to pick up, consumer-price growth in the 19-nation eurozone remains stubbornly low despite years of strong economic expansion.

That is a problem for the European Central Bank, which has spent €2.4 trillion​($2.9 trillion) buying eurozone bonds since early 2015 in a bid to drive inflation toward its target rate of 2%. Despite that giant stimulus, underlying inflation—which excludes volatile food and energy prices—hasn’t changed at all over the period, coming in at 0.7%...