Chris Hemsworth and Natalie Portman have next to no chemistry when on-screen together. There's just nothing that clicks there, and it's easy to understand why - Portman being the culprit. She simply doesn't want any part of the MCU anymore, and hasn't since the release of the first Thor, which translates into a lacklustre performance. Portman first made known her displeasure at Marvel's removal of director Patty Jenkins from the helm of The Dark World (a director she had helped hand-pick), and then came word that she was literally being forced to return for the sequel because of contractual obligation. Rather than coming back because of actually wanting to. Why wouldn't it be a more attractive option to have had someone else in place from the beginning?