"hang-ups"

she has some expectations that say more about herself than about you in my opinion.

maybe her idea of sex is that she must be the object of desire at all times — not satisfied with the fact that you are making love with her (as in merely doing your duty) — but that she's soooo irresistably sexy that your boner must be saluting and standing at attention! where's the heat and passion she reads about or sees in porno? shurely life's supposed to be that way for everybody?

maybe she's a bit unsure that she's genuinely attractive to you and your limp dick is her confirmation of her own fear that she's lacking. Some people are mighty unsure and feel shameful about their "naughty bits" hence her discomfort with this — and probably she's not getting off on your oral ministrations if she's worried about any of this shit

maybe her last lover had a dick always at full mast when he went down and she thinks that's normal for all guys — therefore she feels rejected in some way

I think you can reasure with enthusiasm as you do it, and vocalising your delight and enjoyment as you give her your best, and try positions where she can stimulate you.

Try talking about it with her when you not in the middle of it. Tell her its normal for you as mentioned in the post above. She has a need to know how hot she makes you feel, coz she has doubts about you being into her.