Can't reference sketch dimensions

I have an assembly as follows:

Assembly Skeleton part Sketch A Sketch B Part 1 Part 2 Sketch C (for assembly feature)

In Sketch C I can create dimension references (by clicking on) to dimensions in Sketch A but not Sketch B. Dimensions in Sketch B are not selectable in this context although they are visible and not hidden or suppressed.

Any idea why this might be?

Thanks

Mitch