You may just see lines that are the fish. The arches are when you move over fish or the fish move though the transducer cone. When the distance from the fish moves closer to the transducer then farther away you then get an arch.
If you are moving forward and the transducer is at the back of the boat in 30ft or less you may not be marking fish because the boat is spooking them.
A stick in the water will make an arch if you travel over it. So dont worry about seeing an arch.