My guess - nerves. Sexual excitement and orgasm are only partially under the control of our concious minds (we can't just order our penis up and down at will like our fingers), and the part under the control of the sub-concious can easily be affected by mood, nerves, stress, tiredness...
The nuisance is that, as soon as it has happened once, you are more worried about it happening next time, and that worry then makes it more likely to be a problem!
Try to relax, and make sure the mood is right (easier said than done!). Don't have too high expectations.
If you can make yourself come through masturbation, maybe you could do this while she strokes and/or licks your balls and the bottom of your shaft? If you've done this successfully a few times you could go back to having her try to bring you to orgasm - you'll be more confident that you can come with her being involved in the process, and she'll have seen what you do when you masturbate which may help her technique.
You might also want to try some sort of massage oils, which may increase the sensation for you - you can get ones that make you feel warm, and/or tingle.
First word that's in stars is Oral 2nd is ejaculate sorry i can't seem to Find the Edit.