If you notice threats easier on a computer, that's because you're more used to playing on a computer.
I had the same problem too, but it dissapeared real quickly after I started playing and analyzing more over-the-board. Handily, the solution to your problem is "play more chess!"
I've been struggling with this as of late; I seem to notice patterns and threats much easier on the computer than I do on an actual board. This is effecting my tournament play (otb). Does anyone else have this problem, and how, (hopefully), did you straighten out this problem?