Thank you for asking. I can understand your concern. Not able to penetrate the vigna can be a problem of condition called vaginismus. It is female sexual problem where the vagina is not properly dialated as it should be in normal case. Resulting is difficulty in entering despite of good erections.
Next Steps
You both together must visit a nearby Sexologist. One must learn and understand proper process of sexual responses in males and females.
Health Tips
Don't worry. It is treatable.