When reading your questions my mind went in several directions because there are lots of layers to your situation.  Even though your mother says she is not depressed or you may not feel she is depressed, I would rule out depression with a careful assessment from her doctor.  It would be a shame for her to be suffering from a treatable medical condition that is preventing her from enjoying life.  

In your message, you said you are still working so I'm guessing that your mother is spending a lot of time alone.  Depending on your mother's interests and associations, I wonder if you could set up some outings or visits.  If your mother is belongs to a church or temple, do they have a visitation program?  Are there any organizations that provide visits or phone calls to seniors?  Could you enlist family members or friends in calling or visiting your mother? 

I imagine that you may have encouraged your mother to attend the local senior center.  Keep encouraging her to go out.  Even one outing a week could make a big difference.  What have been your mother's interests?  Could you hire a college student or another senior with an interest in common, to visit your mother regularly so they can discuss music, quilting, bird watching or whatever interest that your mother has.

Many people near the end of their lives do enter a contemplative phase where they tend to withdraw from the world.  It is possible that your mother is engaged in a life review and putting her life journey in perspective.  You can support this by encouraging her to share her stories and record them in some way, writing them down or creating audio or video tapes.  This could also be another project for one of her visitors.

Finally, I'd like to say that each of us is responsible for our own happiness.  You can no more create happiness for your mother then she could for you when you were a child and came home crying after a school day disaster.  Your role at this time in your  mother's life is to walk beside her and support her.  You cannot carry her, she must walk her own path and find her own way through.  A great gift to your mother might be your time and attention, not doing tasks for her benefit but rather talking, listening and remembering together.