Struggling to stop parapet wall leaking

[stoobloo]

I noticed my parapet wall was dripping water down the wall, a crack between the mortar joint and the coping stones was apparently the cause as there were water marks down the faces of the bricks immediately below the joints.

I've ground out (1st time 10mm, 2nd time completely) the mortar and replaced. First time 4:1 ratio of sand to cement. Second time 3:1.

There's still a problem, the water appears to be getting to the DPC below the joints and then making the mortar joint wet - also allowing water to bypass the felt flashing and dampen the inside of the garage roof.

Anything else I can do? Could my cement/sand be the fault? I mixed it thoroughly both times, so don't think it's that.