McGregor explained that when we meet Obi-Wan in the new series, he’s in hiding, which means he’s forced to behave a bit differently than he did in the prequel films:
“At the end of the third episode, the Jedi Order are all but destroyed, and those who aren’t killed have gone into hiding and can’t communicate with each other. So for 10 years, Obi-Wan has been in hiding, he can’t communicate with any of his old comrades, he’s living a pretty solitary life. He’s not able to use the Force, so in a way he’s lost his faith. It’s like somebody who’s stepped away from their religion or something, if you like. The only responsibility to his past life is looking over Luke Skywalker, who he’s delivered to Uncle Owen and Aunt Beru.”
Whether Obi-Wan cannot use the Force because he’s cut off from it or because he’s unable to give away his identity isn’t explicit, though it’s likely that he simply cannot use the Force for fear of being discovered. Surely, he didn’t forget like McGregor forgot how to play Obi-Wan. After all, most of the Force users were hunted down and executed, so tapping into the ancient magic is pretty dangerous to do. There are still Sith left who could potentially feel his use of power, and if he’s only out to protect Luke, then he needs to avoid it at all costs. That would be pretty tough for someone like Obi-Wan, as engaging with the Force is a lot like prayer in our own Earthly faiths. At least that wouldn’t keep McGregor from pretending to use the Force in real life!