Struggling with Transitions


I am trying to get the backgrounds change as a user moves from one section to another as shown by @pixelgeek in this video.

But I am struggling to get smooth transitions.
The transition from ‘Who We Are’ to ‘Why Us’ isn’t smooth as the transition from section ‘Why Us’ to ‘Our Work’. It shows the backgrounds of sections ‘Home’ and ‘Who We Are’ for a split second.
You can see it here.

The read-only and published links are below.
Thanks for your assistance. Really appreciate it.

Kiran K.