Thank you for the query.
is a natural process in every individual , as you said your mother is wearing dentures since 11yrs which doesnot fit properly and hurt her always. Dentures are not preformed prosthesis
they are made according to the size of the patients maxilla
and mandibular process. Possible reason could be the supporting bone doesnot have much retention to hold the denture in place and the age is also a factor to it. If your mother is diabetic ,she is prone to faster boneloss soo the denture size keeps on changing in a year or two leading to ill fitting dentures.Discomfort especially in the palatal region may be because of the impingment on the non stress
bearing areas. So see a prosthodontist for better treatment.
Hope this helps you. Take care.