gradient not applied to entire selection

In the image below, the top area is too dark.  So I made a selection of it, made it lighter with an exposure layer and then tried to apply a gradient, clicking on the bottom of the exposure layer and dragging up.  But the gradient applies only locally - to the immediate area where I am clicking - not across the entire selection layer horizontally.  The spot where I created the gradient can be seen right under the letters "Ch" on the right hand side.  The selection layer has an uneven bottom border - it's higher up in the middle section on the torn paper and lower on the sides on the brick - is this why the gradient isn't working across the entire layer from left to right?  Thanks.