Why is it that every time a young man proposes to me after I have prayed for marriage, the engagement does not materialize following the Istikhara prayer, and what should I do to achieve my wish of marriage?

My advice is to persevere in supplication and in seeking a righteous spouse, while maintaining a good opinion of Allah and being cautious of rushing the answer. For Allah may divert something a person desires and instead reserve for them something better.