AI Controller can't use the Y-Axis

AI build an AI controller in C++ and I use generally the X Axis but the AI should walk a bit on the Y-Axis. I use the MoveToActor function to let the AI walk to a target point or the player but he never walk on the Y-Axis. I checked the NavMesh but there is everything fine. I use only C++ code no behavior Tree or something else because it’s a really basic AI and this would be an overkill.
How can I move my AI on the Y-Axis now? Is there anything I missed?
~Josai.