How to tell: In GIMP, layers don't need to have the same dimensions as the image: they can be larger or smaller. If you try to paint outside the borders of a layer, nothing happens. To see if this is the case, look for a black-and-yellow dashed rectangle that does not enclose the area you're trying to draw at.

How to fix: You need to enlarge the layer. There are two commands near the bottom of the Layer menu that will let you do this: Layer to Image Size, which sets the layer bounds to match the image borders; and Layer Boundary Size, which brings up a dialog that allows you to set the layer dimensions to whatever you please.