This is sort of a bug report, sort of the user experience report if anybody should encounter the same problem: I struggled with a T spline that wouldn't convert. The problem was a singular point, which somehow had sneaked into my surface (not on purpose this time). The point is between the two "3"s below. It does create a minuscule distortion, the curved white streak, but that could be missed, especially in box mode. The bug is that this point is not marked in red in "repair body", even as it prevents conversion. Also, it is not possible to repair the situation in "repair body": the point cannot be converted into a star point, neither will it be deleted.
But manually deleting this degenerate T point, outside "repair body", allows the surface to convert!
Is it possible to send us the model? You can attach it here, or if you wish send it to phil dot eichmiller at autodesk dot com.
Thanks!