It is quite normal to show signs of inadequate trust in a marital relationship, especially when it is just beginning.
This is because trust does not happen automatically in a relationship, or simply because the two people involved are of same fold.
This is also because trust is something based on knowledge and no one can trust someone they do not know enough.
Trust is something that is cultivated over time and, as some say, it is earned.
As the two of them submit to the will of God in obedience to the word, the leading of the Spirit, and spiritual authority, trust in God and trust in each other increases.
Since the main essence of courtship is getting to know each other, it is normal that those involved will find certain hidden traits of the other person issuing forth.
Responses to these traits can be admiration, shock, surprise or utter disappointment, depending on the nature of the trait.
It is also good to understand that since they both are a work in progress, under the Lord.
Thus, the reaction of one party to the weaknesses manifested by the other must be tempered with maturity and faith.
Part of what can cause lack of trust in the heart of a beloved sister towards a brother she is engaged to includes gross dishonesty, irresponsibility, a recalcitrant nature that he is not willing to give up, or a lack of willingness to submit to spiritual authority, among others.
In situations as these, it is best to refer him to his pastor for correction, admonition and mentoring.