Not all aircraft have ADS-B transponders.
From the link below:
"Today, roughly 60% of all passenger aircraft (70% in Europe, 30% in the US) are equipped with an ADS-B transponder. This percentage is steadily increasing as ADS-B is set to replace radar as the primary surveillance method for controlling aircraft."
Common aircraft models that usually have an ADS-B transponder and are visible on Flightradar24 (within ADS-B coverage):
All Airbus models (A300, A310, A318, A319, A320, A321, A330, A340, A350, A380)
Antonov AN-148 and AN-158
ATR 72-600 (most new deliveries)
BAe ATP
BAe Avro RJ70, RJ85, RJ100
Boeing 737, 747, 757, 767, 777, 787
Bombardier CS100 and CS300
Embraer E190 (most new deliveries)
Fokker 70 and 100
McDonnell Douglas MD-10, MD-11
Sukhoi SuperJet 100
Some newer Ilyushin and Tupolev (for example Il-96 and TU-204)
Common aircraft models that usually do not have an ADS-B transponder and are not visible on Flightradar24 (within ADS-B coverage):
"Air Force One"
Antonov AN-124 and AN-225
ATR 42, 72 (except most new deliveries of ATR 72-600)
Boeing 707, 717, 727, 737-200, 747-100, 747-200, 747SP
BAe Jetstream 31 and 32
All Bombardier CRJ models
All Bombardier Dash models
All CASA models
All Dornier models
All Embraer models (except most new deliveries of Embraer E190)
De Havilland Canada DHC-6 Twin Otter
Fokker 50
McDonnell Douglas DC-9, MD-8x, MD-90
Saab 340 and 2000
Most helicopters
Most older aircraft
Most business jets
Most military aircraft
Most propeller aircraft
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