sorry i meant chine on the bottom. thought it couldnt be right behind one
A common issue with any boat hull is that there are certain features constructed into hulls that disrupt the flow of water that is passing by the transom...
If there is any feature of the hull...or even an anomoly like a ding in the hull...this can create bubbles that interrupt the sonar ...
Sonar does not work thru air...bubbles included...
Mounting placement is all about having clean bubble free water flowing across the face of the xducer...
If the chine on your hull creates bubbles...this is definitely not the place to mount the xducer...
Still... Try to post a pic of your entire transom from the rear ...and from each side...with the camera at xducer level...
Rickie