Soul probably isn’t capped depending on the hero.
The message saying ‘same stat’ is based on the stats that your soul spheres are currently enhancing, and these stats can be changed with soul refining stones.
Other than that, your question doesn’t seem that clear, so I’m not sure. If using soul stones and soul evolve crystals isn’t working to increase soul level past a certain point, then that might be a bug, and you should send in a ticket to support.