Are you sure you used the correct mortar? Many old houses used lime mortar which is softer. It expands and contracts with softer old brick. The Portland cement mortar common today does not give. So, when the bricks expand, they tend to crack and flake. In the 1930s, Portland based mortar and harder bricks became more prevalent, but some people did continue building with the older style bricks and lime mortar for a while. Just a thought.
Also, the whole concept of the lime mortar, in use since Roman times, was that the foundations of buildings constantly expand and contract. It is better that the mortar be softer than the brick and thus bear the brunt of cracking, rather than the brick.