Bricks, Masonry, Asphalt and Concrete - Soft Mortar

The home is 36 years old split foyer design built on a slight grade from higher in the back to lower in the front. As a result of prior interior water problems the prior owner had french drains installed outside and inside (cutting through the slab) and there are now two sump pumps. There is no longer any water problems inside the home. The foundation is concret block covered with a sand colored skim coat

In preparing to do a minor repair to some chipping in the skim coat, I discovered that in a few places the morter between the blocks is soft enough penetrate with the tip of a knife. Immediately next to these soft areas, the mortar is hard as a rock. The skim coat is also pretty soft and will loose sand if rubbed vigorously with your hand.

What would be the likely cause of this condition, and what is the recommended and acceptable repair. I would not mind painting the exterior skim coat as it isnt that attractive, but I want to improve the property not make it worse.

I greatly appreciate any guideance that any knowledgable person person can give. Thanks, and I would much rather be fishing.