this is an excellent question. i too have this problem. i was able to start seeing my landings on the backroll after really concentrating on looking for it.
the only thing that helped me was doing mental imagery for it. lying in bed and going thru the trick in my head and trying to see what it looked like - literally play it back so i could see every angle of the rotation and know what i would be seeing, the sky, the water, the boat. then when i got out there i tried to look for those things as i did the trick.
the biggest thing i've noticed about this is that if you don't naturally look to spot a landing, it doesn't happen. meaning half the time i would think about spotting the landing AFTER i'd done the trick and not spotted it. i'd land and go "oh yeah, i MEANT to look for the landing." you start riding and instinct/routine takes over and you forget to practice.
sorry i couldn't be more helpful. im still working on ideas for this myself. but i think the biggest thing is keeping focused on trying it. make a resolution to go out one time and simply work on that one thing. keep it foremost in your mind when you're actually out there riding. after you warm up, do nothing but backrolls and each time try to keep your eyes open and look for the water. ride along and imagine it the trick in your head. then try it. keep doing that.
i don't know about you, but i rarely go out with such focus in mind -but the times i have i've done much better. i think we tend to get out there and just get caught up in the fun of riding. a little extra concentration and thought about it might make the difference.