Politics is about contesting goals and actions, and the subject can be affected by the act of “explaining” it. This leads the study of politics (including the definition of “politics” itself) to be based on essentially contested concepts.
It would be a lot easier if we could do "normal science." Not easy, by any means, but easier. However, many political scientists don’t want “normal science” because it places out-of-bounds the questions that brought them to the field in the first place. The subfield of political theory, for example, is almost completely normative and philosophical. Some political scientists, starting around the 1960s, engaged in a “behaviorist revolution,” patterned on a simplistic view of how the natural sciences work, that divided the field of international relations for years. It essentially involved counting things and looking for patterns. Within its limits, some of it is quite good, and it has gotten better in the “post-behaviorist” period by incorporating a greater sensitivity for the perils of concept selection and model-building for what is sometimes a multi-level non-linear self-reflexive creative process.
I doubt political science will ever be a what is called a "normal" science so long as it involves people studying (and recreating) themselves. Too much of it is art. I can give you an explanation for art based on evolutionary biology and neurophysiology, but it won't help you to be a better artist or to appreciate great art. Some areas–such as the application of genetics and game theory–are more promising because they deal with the structural realities that humans have greater difficulty changing. These approaches sometimes provide a larger context for the interesting questions. Even incomplete understanding can be useful. But then, I’m the sort that believes there is a “real” world than we can understand better (but never perfectly) by critical observation and abduction. In political science that statement is itself controversial. I can’t “prove” it any more than others can “prove” their worldview to me.