It helps to see the object, but it really sound like one of the points is a curve point not a corner point. Use the convert anchor tool and just click on the point to convert it a corner point.
Then if necessary drag out a guide line then drag the points to it, that should create a straight line. (If necessary you can rotate that line)
Also keep in mind the less points there is on the path, the straighter or smoother it will be.