The virtual bayplate option mentioned above is one solution, but you likely have a sequencing problem due to the positioning of the bayplate, backup airswitch or entrance end tire locate photoeye (X020).
First item to check is the position of the machine when it is at home position. In each of the different drive speeds, the machine should position where the track stays are within 1/2" of the front endstop bumpers. The bayplate and backup airswitch positioning are shown in the "typical bay layout" drawing in the installation manual. All of these measurements are critical.
As the vehicle pulls in, they will first get on the bayplates. If they continue forward, the bayplate inputs would turn off, and they should pulse the backup airswitch. If they continue further forward, they would block the entrance end tire locate photoeye. Once they realize they are too far forward, those same inputs should pulse in sequence going reverse. The problem happens if two of those components are able to be blocked at the same time, or if one of them doesn't pulse when backing up.
As an example, a car pulls forward and goes all the way past the entrance end tire locate phototyes. When they backup, they block the photoeyes, drive over the backup airswitch and get back on the bayplate. If the backup airswitch signal didn't provide an input to the PLC, then they didn't go sequentially, and the bayplate can get locked out. The reason they do this is to prevent the rear tires from getting on a bayplate and the wash starting.
Please check and make sure each of these are in the proper position and functioning properly. Most often, I find that the machine isn't getting all the way up against the bumpers, or the airswitch is out of position where they can pulse the airswitch at the same time as blocking the tire photoeye.