One of the most common forms of damp in buildings can be quite difficult to diagnose. It revolves around floor levels.
If your external floor level is higher than your internal floor levels it can bridge across the damp proof course (if you have one) or if you have a solid walled buildings then the damp soil / path can help to trap moisture in the wall.
This can be a result of historical additions to the outside of the house. Often the height of the external ground has been increased by things like:
Installation of pathways
Pavements being re-laid on top of existing surfaces at an increased height
Raised beds being placed against walls
Is there a way out of this? Well the only real way of stopping this transmission of water into a wall is to remove the problem. This means lowering the external ground level by the wall. Ideally you need a minimum of 150mm / 6 inches of difference in height between the internal floor level / damp proof course and the ground level outside.
This can be mean digging out a trench around the house to remove the excess soil / pavements etc. If this causes a major issue externally then you will need to install a drain around the walls. You will need to backfill this with large stones so that there is adequate drainage for water to be taken away from the wall and also for air to circulate around the base of the wall and help to keep it dry.