There's always a reason behind decisions made by older people, especially when it comes to personal care.
Start by asking your mom what's going on. If she doesn't give
you a clear answer, consider the type of bath she's getting. Is it a bed bath, a soak in a tub, or a shower? Then consider her condition. Can she stand or sit? How are her balance and mental status? Does she need much assistance? Could she be afraid of falling?
Fear of falling is one of the main reasons elderly people hesitate to take a shower or bath. Getting in and out of a bathtub, in particular, can be challenging. Making sure that your bathroom is safety-proofed will give your mom reassurance. But you may need to turn to a bed bath instead.
Allow your mom as much control over the bathing process as possible, such as deciding the time and type of bath. Older people often feel that their caregivers treat them like kids, so they choose not to cooperate.
Is your mother modest? This doesn't disappear with age. You can accomplish most of a bath while keeping her covered with a towel, lifting it section by section to wash discreetly underneath.
If your mom is mentally competent and thinking clearly, she has a right to refuse to be bathed. Your task then is to win her over by making her as comfortable as possible, allaying her fears or hesitations.
If she has dementia, you may just need to be firm, establishing a structured bathing routine and sticking with it. You can be firm and friendly, bathing her quickly with as much tenderness and reassurance as you can, explaining what you're doing along the way.
People with Alzheimer's or dementia can be fearful of the sound or feel of running water. Try running the water before your mom gets into the tub or shower to get her used to the sound. Using a handheld shower hose, introduce water slowly, body part by body part, with a gentle spray.
Finally, don't make a big deal out of refusals -- you can always wait for a time when she's more in the mood. If her listlessness continues, though, check with her doctor to rule out more serious causes.