فهمیدن یا پیدا کردن دلیل دقیق. ممکنه به جای it ، the exact cause بیاد
The doctor was able to treat the disease although he could not put his finger on the exact cause.
پیداکردن ( دلیل دقیق )
According to Cambridge dictionary, it means to discover the exact reason why a situation is the way it is, especially when something is wrong:
شرایط غیر عادی. دقیقا مشکل را نفهمیدن