I'm a bit ignorant about how successful or otherwise modern digitals are with imitating sympathetic vibrations, but you can carry out a few tests to discover what differences exist. For instance, put the pedal down and play a big chord, or just an individual note. On the acoustic, you're likely to find that you get quite an unclean sound, compared to a digital. This can be one of the problems with practising on a digital - there is no difference in pedalling before a chord or after one, whereas on an acoustic it will be instantly obvious to the astute player that a much cleaner sound can be produced by pedalling after. You might find that your pedalling is partly responsible.
Of course, putting the pedal down after a chord actually changes the sound slightly on an acoustic, so even if you are pedalling correctly, it may well be that the pedal is adding to the tone of the acoustic in a way that your digital cannot.
Then, things like the tuning and voicing of your teacher's piano, which even if as perfect as possible may well contain minute inaccuracies which, compared to your perfectly tuned and voiced digital, could make the sound seem slightly less satisfactory. And, if your teacher's piano is not perfect in these respects this will make a vast difference. Even a very fine piano will not be as satisfying to play if it is not properly in tune. Tactile unevenness resulting from the imperfect regulation of the action will also have a similar effect - the player 'feels' the piano just as much as he hears it.
Just some thoughts which might account for the differences. As suggested, try a high-quality grand out in a music store and see how the experience compares.