The most critical factor in how a fish finder performs is the installation of the transducer element. Even the best electronics cannot overcome the laws of physics, so proper installation of the transducer will allow performance to meet equipment specifications. Any deviation from perfect starts to impact this performance.
There are several types of transducers; transom mount, thru hull, pocket mount, and in-hull are the primary types. For all of these (excepting the in-hull type) smooth, unobstructed water flow across the face of the transducer is critical and all efforts have to be made to ensure this. There should be no air or separation of laminar flow across the face of the transducer. Air bubbles can blind the sounder to the point that nothing can be seen on the display.
In order to achieve this clean flow of water across the face of the transducer, there should be no hull penetrations forward of the installation. Strakes, intakes, or other equipment that is ahead of the transducer will almost certainly effect the water flow and what can be seen on the display.
With this goal in mind, attention to detail in the fairness of the hull form to transducer face can make a substantial difference in the performance of the system. Any disruption ahead of the element, including caulking bumps or voids, leads to disrupted flow and diminished performance.
Sometimes meeting this goal requires skills only learned from experience. For example, here is a major name brand production planing hull that is not actually flat. It is important to bed any fairing block to the hull to provide a stable installation. Artifacts from vibration and movement of the fairing and transducer take away from optimal performance.
Some installations are more challenging than others and require compromises. This may be having to move to a transom mount because of the trailer, or placement not being in the ideal location due to bilge constraints. It is, however, fundamentally important to minimize any air bubbles across the face of the transducer.
So, if your fish finder performance is not what you might expect, start with evaluating the transducer type and how it is installed. It's the little things that make the difference!