The intruder flies out of the TCAS directional antenna coverage area. The high elevation angles are sometimes called the Cone of Confusion - similar to VOR station passage. TCAS interrogations switch from the top directional antenna to the bottom directional antenna. Aircraft cannot be tracked by the bottom antenna to the same distance as achieved by the top antenna because the bottom antenna always outputs less interrogation power than the top antenna. A monopole antenna does not provide bearing information necessary to display an intruder.
Quadrant crossover occurs.
TCAS might drop an intruder that passes from the stronger forward quadrant into a side quadrant or from the side quadrant to the even weaker aft quadrant. By design, TCAS output power is reduced to the side and aft quadrants because the closure threat is lessened when compared to the forward quadrant. The reported altitude of any intruder is more than 910,000 ft. from TCAS aircraft. Some TCAS installations do not display aircraft which are on the ground. TCAS considers an intruder to be on the ground when own aircraft is below 1,750 ft AGL and the intruder is close enough to the ground to be considered On-Ground.