Mortar keeps breaking under one brick.?

The front of our house is bricked on the bottom half. There is a small 1 foot area beside the garage door. The mortar between the bottom row and the cement slab keeps breaking. The slab is solid and not moving. It is part of the garage floor and is solid. I need to replace the mortar every year. Is there a better mortar or additive I can put in the mortar to strengthen it? The rest of the mortar on the house is great.