Mom with advanced stages of dementia, will not sit still. What's happening?

Constant walking until point of exhaustion. Mom, in her late 70's with advanced stage dementia lives in a memory care facility. Mom is extremely agitated and will not sit down for one minute, without getting up and walking continuously around the memory facility to the point of exhaustion. Mom has no interest in watching tv, crafts, or other people at the facility. Mom has been hospitalized a several times for UtI infections and pneumonia. Mom did not bounce back from the last hospital stay. She is on Risperidone, Aricept, Namenda, and Ativan. The Ativan no longer has any effect for mom's agitation. Taking mom off of Risperidone, is scary, as mom is unmanageable if not taking. It's as if mom's mind and body, will not let her rest. Even at lunch, mom will sit down, than get back up to walk before lunch comes. Mom's doctor is aware and adjusted meds, but it doesn't seem to be helping. What now?