I was viewing the West coast of US from Oceanside down to San Diego on the FR24 map. Noted two aircraft images very close to each other. Seems two images for same flight.

Details 10/13/2018 UTC 16:22 Flight UAL 555 from SFO to SAN Airbus A320-232

Following image suddenly becomes a 'question mark' then disappears.

Curious as to what would cause this.