Thanks Brad. The house was built on top of a grid of I-joists on the concrete foundation. The M1XRF319 and M1XRFTWM are installed in the basement. The farthest sensor is only 25 feet from its receiver. I will move both units to the 1st floor to see if this corrects my issue: on very rare occasions a sensor does not get the signal to the panel. Hoping all that steel is the problem.