Happens alot, even to Ortiz in the beginning. New guy comes along, makes some noise, they scout him out, find the holes in his swing, and he regresses. Just need to practice enough to remove the holes in his swing.
If that doesn't work then it could be a problem with his timing, which is much harder to correct. Classic example was JD Drew. Guy had the most perfect swing you ever saw, but he didn't have good timing so he never became a great hitter. That swing kept getting him big contracts though so he never had the incentive to overcome his poor timing. As such, he never achieved his enormous potential. Has a ring and an All-Star year in 2008 so he wasn't a total bust.