2d elements in the game, like menus and hotbars, are fixed sizes and positions. NPCs are also fixed sizes and positions, but where they appear in the FOV to a particular toon does depend on where your toon is standing, which direction they are facing, and how tall your toon is.
Pretty much like if you and 4 mates were all standing around talking to someone in a bar, you would not all stand in exactly the same spot, nor have the same perspective view of the someone. They might be close, but they would all be slightly different. Unfortunately, the mouse is also a 2d element in the game, and its position does not compensate based on your current view. i.e. if one toon has to move the mouse 15 pixels to hover over an NPC, but another needs to move it 25 pixels (because they have different perspectives and positions), someone will miss out because the mouse just moves the amount of pixels.
So, how to you get around this? Usually by using keybinds to do the work for you.
Interact With Target is a good one for this, as for most NPC's it will then popup the interaction dialog (2d element), which you can then click on. You can also setup some macros for SetView for your camera, so if your cameras are out of sync, a reset to a fixed view may bring it back close enough to work.