problem with mortar in store room

I have a store room that is attached to the side of the house. Two walls are part of the house and the other two are just single brick walls.
The mortar on the inside of these two walls has become powdery and can be just rubbed out with a finger. The outside of the mortar is still solid.
Can anyone tell me what is causing this? The house is about 22 years old.

Is there a product that can be painted/sprayed on to bind the mortar together (similar to wood hardener for rotted wood)?

If not, is the only remedy raking out the crumbly stuff and repointing?

Also, if (as I've a horrible feeling it might be) it's a moisture problem, should I put a waterproofing agent on the outside of the walls?

Any help would be greatly appreciated.