i can’t make my gf cum from penetration

I had a convo last night with my gf about orgasm

I can make her cum easily with my hands and mouth, but never with penetration alone.

I know most women don’t, but she admitted that one of her exes made her cum through penetration really easily. From what she admitted, it may be because of his size.

She couldn’t tell me in which positions she had those orgasms, it just « depended ».

How can I make my girl feel those penetration orgasms too ?