put your finger on sth :
to discover the exact reason why a situation is the way it is, especially when something is wrong.
有事情不太對勁，但你卻說不上來。
這個時候就可以用：I can't put my finger on it.
例句：
● There's something odd about him, but I can't quite put my finger on it.
● Something gives the food a funny taste, but I can't put my finger on it.
