My Girlfriend cannot say "I Love you ! " what does this mean

I am a male in a serious relationship with a lady of 35 years old. I regularly stay with her, have fantastic sexual relationships, travel , share all we have..We have a very intimate and wonderful relationship.

However I am Italian and tell her everyday how much I love Her.. She has once in 3 years told me those magic words, and admitted afterwards to be "incredibly embarassed.. " She cannot/ will not vacalise them..

Is this a syptom of some deeper cause?

We are at the stage of getting engaged, and she desperately wants my children . . . I cannot understand this.

She is Russian.. a lawyer and a very moral, correct professional..
I have never had a relationship like this before.

Many thanks
Nino