My 5th Channel (Spot colour) fills in, but the rest of the image is closecut (transparent).
Example: I have a photo of a Water Filter that I closecut. I then recreated it using a spot colour (plus black), however the spot colour (in this case blue) fills the entire image, rather than just being limited by the bounds of the 4 colour...
Could you please post a screenshot taken at View > 100% and with the pertinent Panels (Layers, Channels, Options Bar, …) visible?
Spot Channels have no connection to individual Layers so you first need to select the area you want to print before filling them black.