I dont think its something wrong whit your equipment, or the model (797) you are using. I have my self get god results whit the same model.
One thing to remember is that wood can consist up to over 90 % of water. That means that the signal can in some cases go right throug the wood. The thing is different if the surface of the wood contains, something elese than wet wood. For example, shells, sand, gases etc..
But if you dont see the wood try to scan a wider area, and you get an other angel, and then you can maybe see the shadows.
Wood, wooden ships, have always been a difficult material to find ewen whit proffessional equipments. Soft wood on a soft seebed can be almost invisible.
But try to tune your unit for best performance. Practise on something you know what it should look like.
In the picture below you can see a broken shipwreck that sunk in the middle 1700 century. In fact there are a lot of wood down at the seebed. But you can still only see the shadows!
Hundred hour of practicing whit theese toys is still only the beginig..
Kimi..