The penis can become erect at very early stage, especially in a young man. She may become anxious as she feels she is keeping him waiting. Both partners may assume she is not responding, when in fact she is. Penis gives a more obvious signal which the vagina may not.
Arousal comes in waves in both the man and the woman.
This is normal. The decline doesn't mean that something is wrong. Control comes with learning and practice. Many women may not have orgasm but are fully responsive. This doesn't mean that they are frigid.