usual suspects is wrong side installed and damage to the rubber as the axle is slipped in. But you carefully installed correct part on correct side and after 3 installs doubt if you damaged the seal all 3 installs.
Best to find out exact axle used to replace the oem and then source the (hopefully) correct part. You can do part numbers comparison with different model year wrx's and why not Sti, you could do either axle or seal. Easy way to eliminate some unknowns about the replacement axle.
Done all 4 of my seals, and both seals on the prop shaft. Only want to do those once every 100k miles.