It could be that it's just not enough stimulation from it. I've only came from oral a couple times my whole life, but that's usually because it comes before sex and I hold off for sex.
Do you know what makes you aroused? Is it the pleasure you get, is it the pleasure you give, is it your partner's face in the act, her body, her sounds? Thinking of what gets you aroused can help bring you to orgasm, as opposed to thinking about bringing orgasm.
Do you know what makes you orgasm? A certain position, a certain rhythm, a certain movement, a certain tightness? Don't be afraid to talk to your partner about what you enjoy. You can also show her by taking her hand and doing what you like. Same with oral, just talk about it before you grab her head and force her to do something. Then just experiment. Try having her stroke you while she performs oral, or fondles your testicle or anus, or licks the glans or crown, or increases pressure, etc. Find something that gives you the most pleasure.
Do you know what happens when you ejaculate? Are you in a certain position, are your legs locked, are your pelvic muscles tight? Pelvic muscles play a big role in ejaculation. I could dry hand masturbate myself for hours if I don't use my pelvic muscles, but if I tighten them I could reach orgasm in under a minute. You can exercise these muscles anytime during the day. Just make sure you relax them as well, because they can lead to PE as well it seems.
