Bath time can be very difficult. They do think they have had a bath... at some point. Since time has no meaning for them, last week can seem like yesterday. Beyond that the steps in bathing are complicated and involve disrobing and being uncomfortable.
Bathing at the same time of day each day, as she always has does help. Don't make it a big deal. Come in cheerful saying..."It's time". Don't ask if she wants a bath because she will tell you no. With Dad I found it beneficial to get him in the shower in the morning before he put on his clothes for the day. While he was sitting on the toilet I would swipe his clothes, coax him out of his underwear, and turn on the water. At that point it is simple for him to step in the tub and shower. It was natural because he was already undressed. I would hand him what he needed, as he needed it. I put the shampoo on his hand and just said... "Wash your hair". I would give him a soapy rag to wash with when it was time. When done I would turn off the water and hand him a towel.
Bathing takes many steps from undressing, to the actually bathing, washing hair, drying, redressing. If you break it down into the individual parts it's easier for them to follow and much less frustrating. It might take a few days to get the hand of this and get her over the frustration but it works better than anything I have tried.
Also make sure the bathroom is warm... too warm for you. Being wet she will be cold and they do not like to be uncomfortable. If she is modest you can put a towel over her shoulders or on her lap. Make sure that the water is the right temperature. Let her check that herself if it helps. Checking the water temp might give her the start she needs to get in the water. Also know that they lose peripheral vision. Coming at her from the back or side does not work. Try to stay in front of her. I have actually coaxed Dad in the shower with his underwear on and then he will take them off because they are wet
I also found it helpful to put a towel in the bottom of the shower for Mom. She felt insecure on the slippery tile floor. Having the "rug" under her feet made her feel more secure. I never argue about the shower. If it doesn't work today then let it go until tomorrow. Try various techniques until you find the one that works.
"Bathing Without a Battle: Person-Directed Care of Individuals with Dementia" by Ann Louise Barrick is a good book to read to help you understand this battle. It covers both institutional and in home bathing techniques but is the best book on the subject. It is a rather expensive book. You can also check with your local Alzheimer's Association who has book lending and pamphlets on Bathing. "The 36 Hour Day" which has been a standard for in home dementia care also has a section on bathing.
Hope something here has helped. Please know that they do not HAVE to have a bath every day. Trying different techniques, and hopefully you will come up with one that works for your Mom
If you have any more questions I will be happy to help.