This and that...
Many times the problem is, fergineer, the complete lack of experience with the TriStar by many of the new(er) pilots and ground engineers.
Many of these folks have not been properly trained, and as we all know, the TriStar is rather unforgiving of inappropriate operation and duff servicing.
The small company I worked for during the last Hajj for example operated three airplanes, all -1's.
The airplane I flew (along with one other Captain) had no 'issues' during the entire Hajj operation, with the exception of high vibration on the #2 engine, which was fixed straight away by one of our on-board ground engineers (we carried three at all times) by trim balancing the number two fan. This one particular guy was straight out of the engine shop in AMM on loan and was very experienced on RR engines.
Due to the excellent record of these ground engineers, I recommended they be paid an extra bonus...and they were, they smiled all the way to the bank.
It is getting harder and harder to find properly experienced folks on type, those that operated the TriStar many years ago, for large carriers....carriers that provided proper training, not just a quick look-in-the-books, and then signed off.
The TriStar is a fine aircraft, and can indeed make profits provided it is operated and maintained properly.