mismatched shading between adjacent objects

Hi, I have an architectural visualisation and appear to be having a problem where multiple objects which are touching, have different darkness of shading, for example a false ceiling and a wall where they meet show a visible contrast in the darkness of shadow, whereas in reality they would flow together smoothly.

I thought this was due to a difference in lightmap density, but altering the value of the lighmap resolution to get the same density visually when checking with the lightmap density view, still gives the same problem.

Is there a trick to getting multiple objects to have the same darkness of shading regardless of their size?