Everybody's different in shape and size, and also how long/short muscle, ligament's and other connective is. So where a technique will work on one it may not work on another. This is why you should have alot of techniques you know, a dozen that you are real good at and two or three you will do without thinking about. Three second rule if it don't work in three it ain't gonna work at all change it.
Also the size difference between you and your attacker can make alot of difference. If they're bigger certain techniques won't work as well as if they are smaller and vice-versa. (though if they are are smaller you may get away with sitting on them )
Also i learn't the arts 'cos i can't run crap at it so i thought better learn to fight, funny 'cos as part of training for tournments e.t.c. for me was to run so now i have an even better choice in my arsenal so if all else fails i have the ledgenary "foot 'o' nike".