I was fishing yesterday and trying to locate 4 rock piles (Purnell's Reef) which is just off OC. According to my chartplotter (Garmen) I was directly over the piles but yet the sonar failed to show these rock piles jutting up from the bottom. I traveled in 50 yd circles but yet could not find the rock piles by the sonar. Does anyone know what would cause this? Thank you, Bob