Maybe she is not getting enough out of it. If she was she would perhaps be more keen to give more. Maybe she is shy or embarrased. Tell her intimately that your aim is to enhance your relationship. Do not make it about you as that can cause resentment.
Have you tried a penis pump? It will help you last longer and also let her know that you are concerned about her own needs. Also get a magic wand for her and use liberally. Hopefully she will be more eenthusiastic after a few successful sessions.