It depends how you created the dimensions originally. If you clicked on the wall edges or on exact nodes.
You could try selecting the dimension, opening the Dimension Settings dialog and ticking 'Dimension Core Only of Composite Walls', under the Dimension Details tab.
If that doesn't work, then you'll need to repeat that step and select your dimension string, and while in the dimension tool, ctrl/cmd click on the wall *edges*.
Cheers,
Link.
