As these drums are open ended as it were perhaps replacing the seals might be the best way forward? The only cases I can recall of black mark faults has always been something caught in between the drum and the seal (hair bands, dryer paper sheets getting stuck, and such)…. HTH?
She did show me a white towel with a black mark in the corner, it looked as though it had been rubbed to me, that’s why I pulled the drum out to check all the seals.
I don’t think she is overloading but the ‘wide open at both ends’ type of drum used in this machine can be lifted a little off the pulleys so it is not a solid fit at the front.