One potential issue is that the seal is not seated perfectly. After you pressed the new seal in, make sure it is sitting at the same distance from the crank`s end all around the seal. It`s an obvious thing, so I guess it`s unlikely you didn`t install it properly.
Another thing: the seal originally is not flush with the engine casing, it`s pressed further in by about a mm. I`m not sure if it matters, but may be a good idea to position the lip exactly where it was before.
If all these all checks out, then another possibility is that the crank got scratched during the rebuild. I hope that`s not the case. Once I had to replace the crankshaft on a different car just because the surface of the shaft had some tiny, barely visible scratches right where the seal`s lip ran...