Over the years there have been lots of rumors and speculation as to what the real problem is. I saw this posted recently on another message board. Of all the explanations I have read this one has the best provenance and provide a very plausible explanation for the issues and for why they haven't been fixed.
"Ok, so back in September I was at a conference at WDW, and the wife decided to book a Dine With An Imagineer lunch at DHS. I skipped one of my educational sessions to do this, and we ended up with a 2-for-1. The main imagineer was a Show Design and Production manager, who was a DWAI veteran (and most recently involved in the Frozen Ever After project); the other guy was a mechanical engineer in charge of show quality, who was a newbie-in-training, at least when it came to these dinners. He explained to us that it was his job to do periodic "reviews" of the rides, and point out areas where show quality is falling below certain standards. He's also heavily involved in maintaining ride systems and animatronics. It was a fantastic experience, and we learned a lot about how things work behind the scenes, but nearing the end of our time, I (obviously) couldn't resist bringing up the Yeti.
Immediately upon my mentioning the Yeti, I could see that it was an obvious a sore spot for him. He stated that there have been multiple proposals put forth for repairing it, but none of the "big shots" have been on board. As for the specific problem, he mentioned that there are a couple of factors: flaws in the original "design calculations" (these were his words), particularly with regard to operational and maintenance conditions on such a large animatronic, and inability to perform proper maintenance on the Yeti. No mention of "shifting/failed foundations" as is often suggested. As a practicing structural engineer, I wanted to know whether this was the problem, and he indicated that the main issue is the animatronic itself.
The other factor is the ability to perform maintenance on the animatronic. I think this is the source of rumors that they "can't replace /fix it without opening up the mountain" rumors, but it's actually much simpler, and this issue ties into the first. He specifically talked about unanticipated stresses in parts of the animatronic due to lack of maintenance in other parts. If one of the motors in the yeti's elbow wears out or isn't functioning properly, but they continue to operate under those conditions, then higher stresses are transferred to the shoulder and chest, etc. My best guess regarding his comments about "incorrect calculations" is that he was referring to fatigue related problems in the robotic parts, and possibly in other structural supports.
The other major factor is that things have changed dramatically at WDW in the last few years regarding their compliance with OSHA standards for maintenance and fall protection. Any new work done to get the Yeti operational means that they have to update the design to meet these standards, so that ongoing maintenance on the animatronic can be safely performed. This would involve major upgrades to allow compliance with fall protection and other things related to maintenance workers.
It was encouraging at least to see how much it bothered him that it didn't work. He brought up the Universal dig re: their Kong animatronic (it moves...) and said that he and his colleagues all read blogs and other social media comments for research and to pick up on things they miss in their reviews. So, there you have it. Take it for what it's worth, but this guy was no bus driver..."
