Most of the time, it's a collection of stuff in the lower trapway.
We sometimes can clear a trapway using a large end closet auger with a 1.3/8" end on it.
Most augers at the hardware store are 3/4".
Not real big, and sometimes they pass right by the problem without pushing it out.
If the trapway is clear, then it will flush on a single try.
We use a closet auger with the large 1-3/8" end on it.
Smaller cables can go right by the obstruction and not move it.