Parker79,
Welcome to the forum. Brush piles can be tricky sometimes. Are these man made brush piles or natural?
The brush absorbs the water and the sonar passes right thru making it very hard to see sometimes. The bottom hardness if strong can hide them also and you will need to look for the shadows of the brush vs the hard return of the brush itself.
What you can do is make a recording of a known brush pile keeping a very straight path. Go slow, say less than 3mph and set the chart speed to around 3. Play the recording back on the unit and pause the recording where you think the brush pile may be. You can now adjust the sensitivity and other settings to see what works best to find them. You can even change the colors. Some colors show objects better than others.
See what works best for you and your area. It will not always be the same, but you will start to learn what settings work best for different situations.
Good Luck,
Robert