Mouse page key does not respond?

As stated above, my mouse page key does not always respond. I recently bought a new mouse with 3 side buttons, which I use for Fortnite. However, the third side key does not react when I press the mouse button 1 (shooting button) at the same time. To react, I have to release the mouse button 1 and then press it, which is annoying when playing Fortnite. Does anyone have ideas?

Mouse broken shortly after purchase = clear warranty claim, so exchange or reclaim money.

I'm not sure because it works, if it is operated alone. Just not if at the same time any other mouse button is pressed.