Positive NFP figures are good for the economy, so investors will buy US dollars in anticipation of a stronger economy in the future. When the NFP is worse than expected, the US dollar falls as investors sell their US dollars.
Sometimes you'll notice negative effects that don't make sense at first. This is especially true when the data released surprises traders. The old adage "you don't trade the news, you trade how people interpret the news" applies a lot to NFP forex trading.