The cuvettes that go with the Spec 20s have an index line at the top that should be lined up with a line on the front of the sample holder. The small plastic cuvettes that are used with the UV-VIS have a small pointer on one side. That side should be put in the holder facing to the left. Be sure to use the same cuvette in the Spec 20 for the blank and the samples you are analyzing whenever possible, but if you must use two cuvettes always use one just for the blank and the other just for the sample. It cannot be stressed too highly that the above rules must always be followed whenever you are using any kind of visible or ultraviolet spectrophotometer.
Sloppy technique is inexcusable.