Modern low-volume cisterns give less flush energy than older ones, so a run with a slight fall problem or a rough patch can struggle. If the property has had drainage altered by an extension, that is the first place to look.
Inside the pan, scratches from a previous attempt to clear a blockage with a hard tool create a rough surface where paper catches. Once that starts, it recurs.
Outside, the usual culprits are root ingress, a displaced joint or a bellied section holding water — all visible on CCTV, all repairable without excavation in most cases.