A theory, developed in conjunction with reader M.V., about why she's had trouble.
Clinton is like an efficiently priced security: the market -- i.e., the voting public -- already possessed an enormous amount of information about her.
Changing your support for a candidate is highly liquid, and that virtually everything that you could say or think about her has essentially been priced in. (Experience, Bill Clinton, strength, toughness, etc.)
Events push her numbers up or down but, at it's root, the overall national market puts her around 40%.
She has probably the least volatile polling track record in recent American history. (Try to find any other presidential candidate in the last three decades with as flat a national tracking number in the 12 months prior to the nomination being decided, or six months, or even three months.)
Second: It's also probably the first time in history of presidential primaries (of either party) that someone with the apparently unflinching support of 40% of voters from the outset couldn't move more than 1 in 6 of the remaining 60% to go on and win.
And these are Democrats!