to neijia
there is a situation most properly happened when push hand with external martial art practitioner: they don't know how to protect their center, but their center is rooted in the ground, no matter how hard you push or pull or throw them on other direction, they just don't move a little. in this sisuation, any technique of taiji is useless. the only way is that you have more strength than his center, and shot dirctely to his center. what you gained from the long pole training is come to use.
but in taiji push hand, every one knows how to protect its center, and to find others' center(守中用中). who finds the other's center first, who wins the game. it's the rule for taiji. but in a fight or application or the situation i said above, even when you find the center, you don't have enough power to break it down, you dont win the game .
it's about 功力(since i think you know the Chinese character and i don't know how to translate this, so i simply typed the chinese).
maybe lack of 功力 is one of the problem why many taiji practitioners who even practise taiji for many years still can't fight.