I've been thinking about this primary season as two primaries - the Southern Primary, and the Main Primary. This is somewhat an artifact of pushing Southern states to the front of the schedule this cycle. The South is the most conservative region of America (with the possible exception of the intermountain West), none of us should be surprised that Bernie has had trouble there - any progressive would. The only surprising thing has been the degree of strength Clinton has displayed there.
The Southern primary started with South Carolina and will be over on March 15 when Florida and North Carolina vote (I don't consider Missouri to be a Southern State). After that we will be entirely in the Main Primary.

Delegate advantage (leader total minus trailer total), data from fivethirtyeight.com through Monday, 7 March.

In the South: Clinton +265
Outside the South: Sanders +66

(the font 538 uses has the 6 and 8 looking almost identical in my browser, so apologies if I switched any)

Sanders has been unable to win any state in the South. Clinton has had wins outside the South. So far her wins outside the South have been small, resulting in few net delegates. Sanders has lost small, won small, but also won big.

My hope is that this pattern continues, allowing Sanders to close the gap and take the lead.