Premiere pro problem

[beyond the blue]

To alter the exposure to a section of a video clip in Premiere Pro I have always made an adjustment layer placed it over the top of the clip in the timeline, clicked on the adjustment layer to highlight it, made a mask with the opacity pen nib and altered the exposure or colour within the masked area only, however this has stopped working for some reason and all of the clip exposure is changing. Could I have altered a setting somewhere or am I doing something wrong? I've spent a couple of hours on this to no avail.