I have a problem regarding interpretation of a multiple regression. For $Y=a+bX_1+cX_2$, the influence of independent variable $X_1$ on $Y$, the dependent variable, is positive (coefficient $b$ is positive). This means that an increase of one unit of $X_1$ leads to an increase of one unit of $Y$, which is consistent with the economic interpretation.
But in my dataset, $X_1$ has also negative values. For those values, the interpretation is not correct anymore. An increase with 1 unit of $X_1$ will lead to a decrease of Y, meaning an inverse relation. What am I doing wrong? Is it better to use the data as absolute values ABS(X)?