Using a layer mask to blend in an image however when I use the brush and eraser, the opacity is not 100% despite it being set to 100% along with flow and fill. Not sure what else it could be?
The first thing to check is that your foreground and background colors are pure black and white. Type "D" to reset them and see if that solves it. If not, check the options bar and check the brush options for the blending mode and opacity.