A lot off cairns on PC are impossible due missing controls and the physics are way off. Stones are falling when on stable display and it is impossible to move the stones to you or away from you, even watching youtube videos and then imitate it did not work. The one in Dublin is the most unstable. (Ubisoft support is out of order.)
Hello there @Monopolist8! Welcome to the forums 
I'm sorry to hear that you're having trouble completing the cairn challenges. You mention that some of the controls are missing whilst you attempt these. Would you be able to send us a video which shows the missing controls, as well as the trouble you're having when attempting these challenges? This will help us to investigate further.
I recommend that you upload the video to a sharing platform, such as YouTube or Google Drive, as this will allow you to post a shareable link within this thread.
Thanks! 
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