Canvas missing on one of multiple parts

In the attached score and parts, the canvas appears behind the page on the score and all parts except for the alto clarinet part. The only different treatment the alto clarinet part has had is to fiddle with the min and max system distances and the last system fill threshold in the Page Style dialogue. I am not sure if the canvas was there before I did the fiddling, but no matter what extra fiddling I do, including saving, exiting MS3 and re-opening, switching between continuous and page view, changing page size, changing canvas colour, it still doesn't appear.