Okay, first I want to admit that I do not know if a general behavioral problem exists with French Mastiffs or not. But the problem you shared with us is kind of a "respect issue" i.e. when a dog does not show enough love/respect for another person in the house. How can you get through this? I would like to share with you from my own experience.
Our German Shepherd dog Alex was brought to our house by my elder brother. As I had no experience in raising a dog earlier, my brother took the responsibility of raising him. But for work reasons, he had to go someplace else distant from our house. So I did all taking care of Alex during his absence. But whenever my brother used to return, he surely liked him more than me even if my brother stayed in our house for two weeks. So I thought to take some measures and discussing with my brother, who was ready to give the leadership to me, I took the following steps in order to get Alex's respect, love and attention.
1)Dogs follow one leader- It is not like the dog does not love you. Dogs love everybody in the family equally. It's that it's only in their nature, like any other animal, they tend to follow one particular leader. So one has to be the leader for his pet dog in order to get his full respect, love and attention.
2)Be the Leader and Take the Responsibilities- If you want to be the leader, take responsibility for him. You can't be a leader unless you look after him properly. The responsibilities include:
a)Take the dog for outing for physical training
b)Feeding, bathing, taking out for toilet, playing with him etc.
c)Passing quality time with the pet et al.
3)Possess the quality of a Leader- It's in animals' nature to request permission or greet his proprietor. When you are the leader, you should act like one. For example, you should not greet your dog when you enter a room; you can pat him a little, look at him, touch him softly etc. but don't go for playing and fooling around with him when you are really busy.
The dog can disturb you to get your attention like barking, playing near your body, etc. Make a strict eye-contact with him. He will understand your attitude and eventually leave the room. This way you can garner the respect of a leader in your dog's mentality. But DON'T EVER PUNISH HIM PHYSICALLY. It won't do any good.
4)Spend a lot time with him- This is a very basic principle in raising a pet. If you spend a lot of time with the pet, they start loving you. That's how the man who takes care of the pet gets more attention than the others.
Make sure you follow these steps carefully. Hopefully you would notice a change in the pet for you.