Not be a smart-a@@ but maybe limited or no fish around.
I salmon fish and rarely mark fish when on the troll - plenty caught but almost never marked. Will mark bait balls, big arches and schools, small to big arches. In deeper water, 100ft or greater, auto gain will not always help your arch occurance on lower end units. Try manual gain, increasing until screen clutter near surface appears...even a little mid-water column clutter is good thing. Where I do mark a lot of fish is saltwater bottom fishing, and kokanee trout fishing due in part to their abundance, and their large air blatters.
I would save my money for new boat if you are planning on upgrade to grady and then I would invest is a "quality" fish finder. 95% of the finders you read about are consumer grade units and not worth spending hard cash on IMHO. Raymarine, Garmin, Lowrance, hummingbird would not be on the quality units list. Fine equipment yes, but not the quality, top notch filtering, powerful tranducer, super adjustable, sounders that give accurate, detailed picture of the water below.
Quality untils would be anything from Furuno and the higher end Sy-Tex units. Budget about $800-1,500.00 for good unit with 1kw tranducer. Even older Color CRT units from Furuno and Si-Tex put modern consumer grade sounders to shame. Color is a must
I like the Furuno FCV 585 or the Si-Tex CVS-833c. Both of these units support 1000watts 50/200 kHz Transducers. If budget forces 600w 50/200 kHz transducer then go with Si-Tex CVS-833 or Furuno FCV-620.