Could be a hardware acceleration issue with the b&w effect. Turn off OpenGL to verify. also make sure your adjustment layer is not clipped/ masked/ repositioned/ smaller than your comp. Remeber: The adjustment layer merely defines the area to which the effect will be applied, the actula processing still happens on the buffer below it. For anything else we will need more info ansd screenshots. We do not even know what version of AE you are referring to.
You could always animate the Hue/Saturation effect for each photo layer and eliminate adjustment layers entirely.
Show us a screenshot of your Timeline panel, being sure to show the properties and layers that you've set up to attempt to get your result.
Thanks, Dave, for that suggestion. I think appropriately assigning different Hue/Saturation values using the keyframes would actually be the easiest, simplest solution and would make completing any changes an easier task than either using an adjustment layer or using opacities to reveal two separate sets of images (one set in color, the other black & white), both of which I had thought of doing.
Thanks, everyone, for your replies.