Because of the way the software renders images, paint and scratch is disabled when paint and scratch layers are present. However, there is a way to override this. The resulting images are not guaranteed to be WYSIWYG but are often good enough for an overview of the layer. This is most often needed to check if all pads are flashed (the flashed pads will appear in contrasting color).
To change this setting, do "Setup/Visibility and Display" and check the box called "Allow on paint/scratch" under "Pads-to-Trace constrast".