Please Help On How To Fix The Leaking Concrete Exterior Wall Of Our New Home?

It is actually a firewall. Not a basement wall. Made of hollow concrete blocks filled and plastered with concrete. No water pipelines inside the wall. Leaks only when it rains. Rain water seems to seep through the joint between the wall and the concrete beam. Flashing seems OK. Wall has hairline cracks; sweats through cracks, but most of the leaks seem to be through the wall-beam joint.
Already tried lining the top of the beam with concrete sloping towards the wall-beam joint. This reduced leakage, but 3 portions along the joint are still leaking.
Is there any guarantee that this problem will be eliminated by painting the outside of the wall with elastomeric paint, say, Drylock? If so, how long will this solution be effective; that is, how frequently will the wall have to be repainted?
Browsed through some related questions here. Some answerers propose using hydraulic cement.
Which of these 2 proposed solutions would be more effective (over the long term) for this problem?