Maybe she’s not talking because she has very different opinions than the OP.
(I am 100% projecting here but throwing out possible ideas.)
Really, I think you should just let her be. There are thousands of reasons why she may not be talkative. It may be temporary. It may be permanent. But your son seems to accept her as she is, so you should too.
Lol, Marvin. My MIL was one of the nicest ladies, good to me from the moment we met. But she thought “polite” meant always filling the air with words. I can be a chatter, but a lot of times, all I could think to do was nod or say, ummm.
Sometimes, all you can do with a quiet person is ask an open-ended question, see where it goes, and maybe let there be stillness. It’s ok. Smile, show approval, and learn to like the calm.
Ok…thanks all. After reading your responses, I feel I’m doing the right thing and I’m doing enough! I remember well being the GF and wanted to be liked. So as long as I make her feel like that…it’s enough. A smile and hug when she comes and goes, always making her feel welcome. Asking a few questions, chatting a bit, then let her be.
And no comments about hugging, I can tell she is open for a quick hug.
This is somewhat different as the quiet GF became the DIL (way before we expected it!).
She was uncomfortable and quiet with us in the beginning. She was also very young.
I have “bought” my DIL’s conversation with experiences that we have together.
She was 19 when we met and is now 24. S is not a talker. She was uncomfortable around us and he was no help.
I literally wine and dine her with shopping( I buy) and pedicures and so on.
This has given us a number of things to chat about and remember together.
She feels special when she visits (4 hours away) and we have a good enough time.
I think that often having experiences together can both break the ice and form a future
foundation. So DIL is lovely. S still does not talk 
.
DIL is not a hugger but will after a longer visit. I always hug both and say “I love you”
(remember this is a marriage with a G’Son). She hugs me back but says nothing.
Her own mother is cold. But it feels right and ok and she makes the big effort to have them visit.
I really do just love her but it has taken a lot of work to make this a positive relationship.
My own MIL is a nice person who I sincerely do not like. She never made an effort to make me feel
special and she clearly favored her first DIL to the expense of the other 2 DIL’S. I have no memory of ever doing anything alone with her.
All of this is helped by D not being the least bit jealous and also trying to engage with awkward DIL when she is home visiting at the same time…
But when I take them shopping together I have learned to put a price tag on their purchases!
Good on ya, @conmama !
Sometimes, people can be very private/guarded about themselves in areas many of us may not be. Others may be worried about saying the wrong thing…especially around a parent-in-law.
Still, others may find the parent-in-law’s stated opinions/worldview to be diametrically opposed to their own and don’t want to cause issues by arguing.
Especially if said parent-in-law is very overbearing in the “this is the way it is” authoritarian type tone and the child of said parent-in-law agrees or is not inclined/too intimidated to “stand up” for his/her SO so s/he could have some conversational space to disagree/debate reasonably.
Others may just be shy, naturally prefer silence to much talking, or don’t agree with the worldview that one must fill the air with words constantly or regularly*.
Let them naturally engage with you in conversation which could be facilitated by shared experiences, asking them about themselves, researching and asking open ended questions about their interests/hobbies, using light humor(after observing and assessing the receptivity of the individual concerned), etc.
As you may have noticed, I’m very much the black sheep in this respect. Especially considering I lean more towards the extroverted side. 
My MIL was a chatterbox. I think she had low self esteem.
Honestly, the more she talked the less I talked. There should be a natural balance between silence and conversation.
Unfortunately, many people…especially extroverts in our society tend to be very uncomfortable with even short pauses in conversation, much less periods of silence.
It’s a phenomenon which is often noted and discussed among social scientists studying conversational styles along with immigrants, international students, and tourists from societies where such pauses/periods of silence are considered a natural part of conversation/interacting with others.
OP gets it, said so in 22.