For me this is happening from a long time ago, the first and the second half of the season suck, suck really hard. Something you can say is that finally in this second half we have something a little bit new, but before i talk about that.
I have to say that after season 10 or 11, we can only say that some episodes are good, because the rest almost every one of them are bad in some aspect.
Second i have to say that while in season 14 we have only some moments where the light was in Brass getting some action moment and burn off moment because the job before he go out.
Here in season 15 we have in the second half of the season, a lot of screen time for Nick, yes he is not doing a shit, but he looks solid, he looks good and we finally have him be the old CSI that look the evidence, try to find the clues and solve the problem.
For me if you are a Nick fan this second half of season have some good episodes to see him do some old CSI job and take the screen time again in his arms.