I can fall asleep almost anywhere. I routinely fall asleep in the break room at work. Once, I was helping a friend fix his car, and I fell asleep on his garage floor when he went inside to get water.
But in a hot metal tube tearing through the sky, with my neck all kinked? Get out of here, man.
If it’s hot in the plane, that might be blocking you from sleeping. Personally I’m always cold in planes and fall asleep relatively easily both on planes and elsewhere.
It also helps to be jet-lagged. Knocks you out right quick :P Only applies to return trips though.