A couple of my problems with my scottish terrier?

I have a gorgeous little scottish terrier called Radley, he is quite large and very handsome. He is going to be three in December and he is quite obedient at home. I have taught him many tricks which he does perfectly with no hassle, although he doesn't have much of a drive for treats or toys! He is very obedient usually and he is very friendly and lovely, but when we are out on walks he often does not listen to me. He doesn't always do the tricks for my mother, but does them for me. He comes back to my mother on walks but not me. Why does he not come back for me?