Why does Hillary Clinton tend to under-perform expectations?

[JRP1994]

In 2007, Clinton was widely seen as the frontrunner to clinch the Democratic nomination with relative ease. Not a runaway frontrunner, but a clear one. What happened? She got outmaneuvered, outflanked, and flat-out outdone by Barack Obama.

In 2015, Clinton was seen as a massive frontrunner to perhaps sweep all 50 states in the primary and win the general election with relative ease. While she's very likely to be the Democratic nominee this time (sorry to those feeling the Bern), she's having difficulty putting away a 75 year old self-identified socialist. He's won over 1,000 delegates and 15 states.

My question is, why does Clinton have difficulty living up to expectations in an electoral sense? Is it because people set expectations too high? Do late-deciders break against her?  Is she a poorer candidate than people credit her?