Why is the US listed as a “flawed democracy” in the Democracy Index?

I was reading Wikipedia's Democracy Index and it shows the United States as a "Flawed Democracy". Reading in more detail, the article points out that the US has low scores in "Functioning of Government" (6.43) and also in "Political Culture" (6.25), but I don't understand those terms.

Why is the US listed as a "flawed democracy" in the Democracy Index? How should I interpret that?

I guess my real concern is if there is a problem, then I would like to understand it first, and then somehow vote to improve it.