Is it that she won't swallow it, or won't finish it with her mouth? Maybe your jizz tastes bad. A friend of mine won't swallow her husbands, because of medication he has to take. She has tissues by the bed, when he pops his load, she politely try's to make it not so obvious to what she's doing, and spits in into some tissues. If it's meds, you can't do anything about it, if it's your diet, you can try and change it, and make it easier for her to take. If it's the slimy texture in her mouth, get it to the back of her throat, and teach her how to gulp immediately. If it's just the thought of, how perverted and dirty doing that is...like only a whore would do that, then you have 2 choices, accept you will never drop a load in her mouth, because she's just to uptight to change, or smile, and say oh yea...that's why it's so much fun, and remind her, she's not just any slut...she's YOUR slut. Then, give her your best "lets fuck" grin, grab her ass, and tell her, "daddy wants to play hide the salami".
Or, just grab her head, and make her do it.