Jeff, I've run the 620 for 4 seasons now and don't have the issues you describe. Here's a few things you can check:
Pressing the gain knob, gives 3 choices, fishing, cruising, and off. The auto gain must be "off" for any of your manual changes to take effect. You can offset the gain for each frequency. I boost the gain on the 50 kz low frequency side, almost to the point of having a cluttered screen, and keep the high frequency side at "0".
Check the interference settings. Turn it off and see if you have any electrical interference on the screen... use the minimum supression needed to keep the screen clean. Same thing with low level noise and weak echo settings.... turn off as much of the auto filtering crap as you can to get a true picture...... dump the fish ID icon setting too.
Remember, you are looking at a 3D world on a 2D screen.... what you see on the screen depends on how a fish swims through the cone angle. Very seldom do I see that perfect "arch" of a fish ... more often it's a blob, line, or streak on the screen.
Check out this link for more help.
http://www.thehulltruth.com/marine-electronics-forum/196651-i-need-help-operating-my-furuno-fcv-620-using-sonar-general.html