What is the reason for a girl's delayed marriage, despite the marriage of those younger than her, and the failure to complete the marriage after engagement, and is there a solution to avoid spinsterhood?

Marriage is a provision decreed by Allah. You must exert the means [to achieve it], the most important of which is supplication, and searching for spouses with the help of trustworthy relatives or friends. If you fear that there is magic or an evil eye hindering your marriage, then you must perform ruqyah al-shar'iyyah (Islamic exorcism).