Each of the three elections in which the leader in the pre-convention polls went on to lose had differing scenarios.
In 1992, George H.W. Bush led in both a two-candidate race vs. Bill Clinton and a three-candidate race vs. Clinton and independent Ross Perot prior to the Democratic convention that year. Perot dropped out of the race during the Democratic convention. Clinton enjoyed a tremendous bump in support after the convention and never trailed again, even after Perot rejoined the race in October.
The other two elections in which a leading candidate eventually lost provide clearer examples of changes in the overall political dynamic caused by the convention/debate segment of the campaign. Entering the conventions in 1988, then-Vice President George H.W. Bush trailed Democrat Michael Dukakis by six points prior to the Democratic convention, and fell further behind after it. Bush’s lagging in the polls might at least partially be ascribed to “party fatigue,” stemming from the same political party’s holding control of the White House for two terms (all four incumbent vice presidents running for president since Richard Nixon in 1960 trailed their opponent at this point of the campaign). However, a month later, Bush received a big bounce from his convention that pushed him into the lead, and he never trailed from that point on.