There are a numerous reasons why a toilet is not flowing. 3 main reasons are:
1. Blocked main drain caused by broken/cracked pipe which is allowing waste water to escape into subsoil and attract tree roots.
2. Pipe is not flowing in a downward direction known as Back fall. This will then hold water in the drain and all solids will build up and block the drain.
3. Foreign objects have been placed down drain and lodged in Gullies, bends and junctions. Other reasons could be offset Pan connections and faulty installations.