Talk to her & find out what are the elements of the unreadiness. Make sure when you talk, talk the language she can comprehend, not the language you can comprehend and think she can comprehend.

It could be: -

1) Fear of responsibility & being tied down to motherhood and parenting?
2) Fear that with the children, she could be no longer as carefree as now? She may need to give up certain hobbies or outing.
3) Perception that female who married early are usually not living their life to full potential. The peer of the same age not yet married, why she needs to marry now?
4) Fear that your treatment to her will change after the marriage? After marriage, no more coupling stuff as you have achieved your goal. You will change to worse?
5) Fear that after marriage, there will be physical changes to her body which makes her look no longer attractive as now?
6) Fear that she is walking a very traditional & predictable life path, that is the end of her life story?
7) Fear of taking the new role as wife and as daughter-in-law - need to deal with husband's family?

People always say the biggest worry of a woman is marrying the wrong husband.