After a while bricks to become brittle and flake off. This is just an aging thing. I do not know how you can restore bricks and keep them from doing this.
Now, you know that you can get some mortar and some concrete coloring and experiment with this coloring to come up with a close match to repair corners that have flaked off.
Or if you have some bricks the same color, you can carefully remove the mortar and add a new brick in that spot. This has to be done very carefully and your new brick will probably stick out like a sore thumb
Contact a mason in your area and have him/her take a look at your situation and give you some on site suggestions.
This is pretty hard to issue a panacea over the Internet.