James, there are 3 major factors that influence the visability of DL watermarks.
First, many of the matermarks were lightly impressed by the dandy roll making them more difficult to see.
Second, as pointed out by khj, the quarter sheets will have 9 horizontal row of 10 characters. With this spacing some stamps will have just a small amount of the watermark.
Third, if the stamp has a heavy cancel it can mask parts/all of the watermark.
Below is the quarter sheet layou, as you can see the stamps at teh top and bottom of this example show complete or mostly complete letters. Some of the stamps in the center of the pane show just a small portion of multiple letters. The letteres are approx 16mm high.
Below is the actual watermark on a block of 6
