Ahh the joys of the vegas masking tool. try click near the last point you made, it should reactivate the path if it doesn't but the tangents reappear try click on one of the tangents, it it turns into into a curved one click it again to change it back, hopefully by now you should be able to add to it. What you want is for one of the end points to be yellow once that happens you can continue chaining a mask.