Used to have the same issue all the time. I'd alternate every week between Arriva, Go-Ahead and Stagecoach and every week Arriva refused tickets that weren't theirs, Go-Ahead refused tickets that weren't theirs (Deptford were notably the worst), only Stagecoach I never had an argument with. Always stood my ground until the driver gave in. Eventually got sick of the arguments, stopped using the bus and went in the car instead. Since I became a driver with Stagecoach, I regularly accepted Teesside Trippers in Hartlepool (not valid) because I couldn't be arsed with the arguments but point blank refused Arriva's East Durham tickets. Never refused an explorer either, all I need ed to see was the price and I knew immediately what it was.