When there is a change in behavior, and the individual with dementia cannot explain this change, caregivers must initially check to see if there is an acute condition precipating the
change. Perhaps your mom is experiencing pain or discomfort from a medical problem such as a urinary tract infection. Her physician will be able to rule out any illness and then look at her this new phase behavior.
Alzheimer patients are known to either be very awake and restless or lethargic and sleepy wanting to stay in bed all day and night like your mom. It is common for those with advanced Alzheimer's to become less mobile and sleep more. Since their ability to understand directions and clues to perform action progressively lessens, caregivers must modify their communication to be more tactile and less verbal. Thus, if you want your mom to come to the table to eat, you (and another perhaps) may need to physically transfer her there rather than asking or negotiating with words. Like you say, when she is at the table, she is hungry and thirsty but when you ask or cajole, she resists. It sounds as though she needs you to make the decision for her at this time. Often putting a glass of water next to her without asking if she wants it will result in increased drinking.
If her appetite does not improve, her doctor can recommend stimulants or supplements.