Was the main sewer at a higher elevation in relation to the bottom of the catch basins? This type of layout may be necessary because of the local land topography.
Take, for example, three catch basins linked in series that dump into a main sewer. Each catch basin at a higher elevation than...
I have this problem every now and then, too. I usually set the TRANSPARENCY command to ON for each image. It works pretty good for me, but I still need to regen alot.