It's been the story of all three of her reelection runs: Barbara Boxer always looks vulnerable early in the cycle and then pulls away and wins easily in the end. It looks like that will be the case again this year, as she now leads Carly Fiorina by a 52-43 margin.
It's not that California voters particularly like Boxer- in fact by a 46/44 margin more of them disapprove of her performance than think she's doing a good job. But Fiorina simply has not proven to be a formidable candidate. 49% of voters have an unfavorable opinion of her to 38% with a favorable one. It's not impossible to win as a Republican in California, but it is impossible to win as an unpopular Republican.
Fiorina would have had to do three things to win this race: win independents by a large margin, get close to 100% of the Republican vote, and get a significant amount of crossover support from Democrats somewhere in the 20% range. On the independents front she's doing well, holding a 54-37 advantage. But she doesn't have nearly the amount of bipartisan support she would need to win, getting only 10% of Democrats. That's actually less than the 13% of Republicans supporting Boxer, which means she's not holding onto her own party's support to the extent she needs to either.
Perhaps if the Republicans had nominated someone different they'd be in a stronger position to win this race but barring a major movement in the final week it looks like Boxer will be back for another term.
Full results here