That's how mine used to be. I got her a 7 inch long and 5.5inch around dildo. Her first reaction was that it was too big. We tried a few times and had to be pretty gentle. After three or four sessions with it I noticed she was taking more of it. I made a few comments about how wet she was and how swollen her lips were etc and she seemed surprised. She can now take the whole thing and for the first time ever the other day, she initiated me putting it in her. I can't fuck her hard with it yet but she can ride it balls deep.Trying to do this but she's tight and prefers her vibrator and says the normal sized dildo is too big. Thinking of getting a sleeve and hoping she'll go along with that
In her case, I think it was more the mental side of it. Once she realised (with some encouragement) that she could take it all then she learnt to enjoy it.
She still prefers a vibrator but dildo plat has certainly improved.