Sometimes there is a problem that wiull leave out the T/D. This will also give you misleading info on fuel usage, like tell you you will have the same amount of fuel at the end of the flight, as is currently on board at the beginning.

Two things I have found.
1) I can't mess with the STAR don't change speed or altitude constrains as this leads to no T/D. It is often okay to change the TRANS after the STAR though.
2) The STAR or TRANS is not seen correctly by the FMC. By this I mean, there are no transitional speeds/altitudes in the STAR/TRANS and so there is no T/D.

A couple of things I do.
Add my own speed/altitude constraints, but I must perform my own descent, by using Descend Now (DES NOW) on VNAV page 3 or FLCH Switch to do that.
I look for a STAR that is almost the same as the one I am using. Like for YSCB, POL1A seems to not be recognized, but POL1U is. It's weird because POL1A.ARRAN and POL1U.ARRAN are identical.

Mark