Brush opacity not working in layer mask

Hi, Hope someone can help if I describe my workflow problem. I create a duplicate layer then press C and T to transform then make a selection either lasso or rectangular to say, enlarge a or move part of the photo, then add a layer mask and select brush to blend in the edges. it has always worked fine until all of a sudden no matter what brush opacity I set it still shows the edge of my selection albeit only slightly.