I'm also experiencing negative CI problems for the first time ever. Over the past half dozen games I've always managed to get my CI rolling up quickly. For me the culprit seems to be a 757 that I leased at 65% condition and its rapidly dropped to 50% despite all regular checks being applied. Normally I don't buy/lease anything below 80% (and given this experience, never will again), unfortunately I have no cash on hand to repair it. Curiously, when I looked into doing so, it was going to cost $300,000 less to repair it to 71% than it would have cost me to repair it to 70%! Is that a bug or some mechanics quirk I need to know about. I can report this to the bug thread if, as I expect, it is a bug.
But two questions: do a/c in mediocre/poor condition deteriorate faster than those in good condition? And how big a factor is age of the airframe? Clearly the older the frame the quicker the loss of condition. I ask these because in my previous experience an a/c that starts out at 95% and is less than 10 years old rarely drops below 85% between B checks.