I had the same problem as you describe with my new installs a few months ago. First turn on each individual step of your wash manually and make sure that there isn't any water or chemical spraying up into the sonar. Mine was getting sprayed each time the hydro flex option was used. One of the jets had a fingernail shaped piece of metal inside which was deflecting the chemical and water right at the rear sonar. This was causing all kinds of problems. Second, Go to Home Depot and purchase a 4x8 piece of 3/4 plywood or sheathing and lay it on the floor in the area where you want the rear sonar to detect the end of the car. This wood will be an indicator if your rear sonar is getting a good enough bounce off the concrete floor. My floor had water pools (very small ones) in the area of the back bumper of cars which caused problems with the sonar bouncing back up.
So: manually turn on each and every step of your wash to see if the sonar is getting sprayed for some reason. Lay wood on the floor. If the wood solves the problem then purchase a 4x8x 3/16 sheet of steel and anchor it to the floor in that area.