iTunes 12.9.0.164 Crossfade not working
Here are some steps I followed to get crossfade to work in iTunes:
1. Prefs>Playback>check crossfade box
2. To the right of the volume control at the top of the iTunes playback window, try selecting "computer only" (deselect any airplay devices, i.e. airport express)
3. Quit iTunes and relaunch.
This worked for me. When I added in airport express playback, it had effect on the computer volume and crossfade no longer worked.
Hope this helps
iMac (Retina 5K, 27-inch, 2017), macOS Mojave 10.14.1