Apparently the bill has had a "hold" placed on it by Sens. Coburn (OK), Enzi (WY), and perhaps
Senator DeMint (SC). This prevents it from coming to the floor for a vote. But as I understand it, the Majority Leader, Sen. Reid, can override the hold and get it to the floor. So send an email to him, too.
Several of my cancer parent friends were in D.C. last week, speaking to the congresspeople (or their aids.) We're all getting an intense lesson in how these things work. It's not as simple as Schoolhouse Rock taught us.
What we've learned: Sponsors are the ones who write the legislation. All a co-sponsor does is
pledge ahead of time that they will vote for it. There is no work or anything required to be a co-sponsor, it is just a pledge of support. So, by not co-sponsoring they are not pledging a vote so to speak. Most likely they are undecided or against (or possibly just too lazy to ask their aid to add their name to the co-sponsor list). The other reason co-sponsoring is important: the more co-sponsors the bill has the more obvious it is that it is bi-partisan and non-controversial.
In any case, as Jess says, write to everyone. Curesearch has a handy thing that will automatically draft a letter to your senator, and will note whether they are are co-sponsor or not.