There's a lot of things that come to mind; comfort level, technique, environment, timing. Are they doing it when they want to or when you want to? When you're comfortable and want to orgasm.
When you masturbate alone, how long does it take you? Do you use vibrators or other aids?
Do they? Are you recreating the setting, comfort and desire when you are with this other person?
Sorry, lots of questions, but it seems like more information would make it easier to help.
Rogelio
Guess I should say I can give myself orgasms but they cant..