Sanders is doing terrible amoung late deciders.
That is literally the reason he is “underperforming”
Technically, he isn’t even underperforming
He is matching his poll numbers
Others are simply over performing
This is something I identified last summer and is still true, although things didn't sort out as much as I thought they would until late January at the earliest.
The largest bloc of voters in this primary election support neither Biden nor Sanders, but have no strong preferences between the various other candidates. They've been drifting with the trends between all of the other candidates: O'Rourke, Harris, Warren, Buttigieg (twice) and Klobuchar have each had their time in the sun with these voters over the course of the campaign. Currently, these voters are divided up among Buttigieg, Klobuchar and Bloomberg (in national polls), with a relatively small rump remaining with Warren and some also with Steyer in select states. It is as-yet not completely clear which candidate these voters will coalesce behind, but as the field winnows down, they can be expected to settle on a single candidate. And that candidate won't be either Sanders or Biden. Given the inability of Buttigieg and Klobuchar to knock each other out of the race, I am increasingly of the opinion that it will be Bloomberg, who has the resources to outlast them, but we'll see. (The one reason this might not happen is that there is clearly a group of voters who will vote for any tolerable woman candidate (by tolerable, I mean not Gabbard), so Klobuchar's support might not completely evaporate.)
