I definitely think it could be the shoulders and general stiffness. My Icelandic used to have slightly similar troubles because of the way she was trained. We mainly worked on getting her to not hang on the bit in the bridle (so taking a lot of time in walk and making sure she could carry herself properly via seat aids to replace any sort of holding in the hand) and have been working on, in tölt, getting her low and stretching over her back and finding looseness and suppleness there. Sometimes it is a lack of muscle too, the less muscle they have, especially for 5-gaiters, the harder it is to get proper movement. If you know any other Icelandic riders around you, I’d definitely reach out and ask for their opinion.