2-year old trying to nurse mom obsessively

We have a 2-year old puppy (hardly a puppy anymore), who has started something that looks like obsessive nursing with her mom! Obviously nothing comes out, and she even gets tired trying to nurse the mom, - but she doesn't stop. It's also becoming a problem for the mom, who hides from her and is even getting red because of the attempt to nurse.
   
Any idea why she would be doing this? Is something missing in her diet, or what's going on?
   
Thanks!

Talk to the vet. Your dog may have Obsessive-Compulsive Disorder. Many dogs get this. There's a drug treatment for it, & other ways to go.
   
Think back. When did this new nursing begin? What happened at the time? Was there a separation, whether the 2-yr-old from the mother, or from your family?
   
Something started this; there had to be a trigger.