All brick and stone expands and contracts as a result of temperature fluctuations over the course of a day and season to season. To account for this, the mortar must be softer than the brick or stone. If expanding stone/brick can't compress the mortar, stress causes cracks to develop and spalling to occur. This is why its common to see spalling on old brick buildings that have been "pointed up" with modern mortar. Old brick wasn't fired as hard as newer brick, and the mortar used back then was correspondingly softer.
I'm no mason, but from the pictures it looks like a relatively hard cement may have been used to construct this wall, which is not allowing the stones to expand sufficiently. But again, someone more qualified than me would need to conduct an inspection on site to confirm that. If that is your problem, then unfortunately there's no easy fix. But moisture compounds the issue, so as others have suggested, taking steps to seal things up and prevent water absorption could be of benefit. Good luck!
I'm no mason, but from the pictures it looks like a relatively hard cement may have been used to construct this wall, which is not allowing the stones to expand sufficiently. But again, someone more qualified than me would need to conduct an inspection on site to confirm that. If that is your problem, then unfortunately there's no easy fix. But moisture compounds the issue, so as others have suggested, taking steps to seal things up and prevent water absorption could be of benefit. Good luck!
