Why are my FPS not arriving - Fortnite?

I have the following problem.

I play on a 144hz monitor

and set 144hz in Fortnite. (Have the FPS display and always have

constant 144fps)

My problem is that when I go into the settings on my monitor it shows that Fortnite only sends 60 fps. (I activated and deactivated V-Sync).

For example, if I now play COD: Warzone and then in the monitor settings my 144 Hz is displayed.

What should I do?

Set the graphics settings very low…