Here is how the tail looks under the skin. You can see here that the tail is very similar to the neck in that it has the 4 wire cables - which are actually only two cables that run through a motor box inside pleo. One cable does the sideways movement and the other does the up down movement. They also have round disks that support the cables and provide the framework for the tail shape. Unlike the neck these disks are solid circles of plastic and so are less prone to breaking. So if there is no resistance to the tail when you gently push against it I would think the cable may be broken.
The one pleo tail I have repaired didn't need the cable fixing but it had a motor that wan't working causing the tail to have no up down movement. I did do a photo video of opening up that end of the pleo because I was also working on the leg at that end which had a loose wire that needed reattaching. Let me know if you decide to try to repair it yourself and I will post the link to the video for you. It may help. Replacing the wire will be the same procedure as replacing a neck wire but getting to the gear housing is a bit different as a pleos bum has different contents to it's shoulders.
I would recommend that the first step would be find out how long the warranty lasts for Amazon. I would think that if he is still under warranty then they would maybe replace him for you. I wouldn't think that they would be able to do repairs as Innvo are the only place that do those that I am aware of. Not sure if your warranty will cover the damage or not as it seems to have been caused by repeated pushing of the tail into a wall.
If you just tell them that the tail is no longer working properly but not how it happened you may have more luck.
She is out of warranty with Innvo as like RWM says Innvos warranty is only 90 days. That will be why they have told you to take it up with Amazon.
Your other options are to live with it and see what happens, repair it yourself, or pay to have Innvo repair it for you.
Try Amazon first and see what they say.
Good luck and let us know what happens.
