Dear Traveller,
The fact that you had asked her once before indicates your desire to have her approval before going ahead. If you have not yet asked her a second time, you would not know if she were ready to give her consent. If her response is not favourable, would you still go ahead with the engagement?
I would suggest talking to your mother about what she thinks of your boyfriend and your desire to settle down. Find out if she has any reservations about him. You didn’t mention the following areas, which could be the cause of her reticence. For example, is he of the same Christian faith? Has he demonstrated sufficiently that he loves you? Is he is from a different country/culture and have you discussed how this may affect your marriage, and where you will live?
If no strong disagreement presents itself, speak about your hopes to be engaged soon. Listen to her views about these areas including how she thinks a marriage might affect her, and how it might alter your availability to her.
If there are concerns on her part but you still feel strongly that you have chosen wisely, then try to reassure your mother whilst stating your determination to proceed. If need be, engage a respected relative or friend to talk with your mother. And if after all of these she is still not convinced, then state as calmly as possible that though you know her views and even her concerns, you will proceed with being engaged and finally to marry this man. You hope that she will respect your decision and that in time, you both will remain close as mother and daughter.