Then there's something wrong.
- Your monitor/TV is broken
- You don't save the changes you make in the drivers so the game runs at the same fps, even if you think you changed it
- Your card is always locked at the same fps no matter what you do
- Your FPS counter is bugged and reports wrong estimates while your game always runs at the same fps
- You get frame pacing and stuttering instead of consistent frame rate
- You think frame rate is something else and even though you feel the difference, you can't put your finger on it
- You have some rare eye condition
Has to be one of those or a combination of more. There is no other way for a person to not be able to notice the difference between 30 and 60 fps.
To be clear, I can tell the difference in "experimental conditions". Having refined my switching technique so I can switch FPS with less screwing around, I can now actually see a difference in game.
What I can't do is tell a difference if it's not instantaneous, and I certainly can't if I'm not looking out for it.