It could be any one of several things:
1. The seal was not replaced
2. The hub of the damper is scored enough that the seal won't hold back the oil
3. The outer metal ring was not sealed into the timing cover or one of them is distorted
4. The timing cover does not center the seal on the hub of the balancer
5. The bolt holes in the timing cover are dimpled to the point they prevent a good seal against the face of the block
6. The front pan gasket that seals with the bottom of the timing cover is leaking
7. The front lip of the oil pan is not making good contact with the front seal because it is badly pitted or distorted
8. All the bolts simply need to be gone over with a wrench and torqued back to specs.
