My impression is that you in some cases have to enter the desired altitude and speed for the last waypoint, otherwise the TD is not computed. As you mentioned it works great for some transitions and arrivals, but i believe it only does if there is a real altitude/speed restriction.. even if there are restrictions for Below(B)/Above(A) it will not compute the top of descent. I'll assume that this is a bug.

I have no idea why the TC is not computed.. maybe the FMC preflight is not complete? I always ensure that the FMC writes "preflight complete" so everything is for sure done.

/Bo