As others have said, it's kind of an unfamiliar thing for beginners. It comes slowly but surely.
Just as a suggestion, why not take a really simple song like "A horse with no name" - just two chords over and over. And play it over and over and over. And just try to glaze over a bit and get into the music (illicit drugs can help but are not required
) and then try syncopating a bit. Or not even syncopating yet - just try changing the emphasis on one of the beats. And then morph that change into another beat. Then try to chop one of the strums a bit shorter than the others. I think you get the idea. It can't really be forced. It's just a familiarity thing.