She’s afraid. Alzheimer’s patient’s have issues with depth perception, the shower may be harsh and feel like pins and needles in her skin. You may do better with a hand held shower and a shower chair.

We have decided her bath days are on Sunday and Wednesday, other days she gets sponge baths seems to be working

Try a different time of day. Right away in the morning, my husband is too groggy to motor through it. Evening would never work - sun downing. Mid day has worked a couple times. (does not mean it will work every time!

Dont make it a battle as the more you try to get them in the shower the more they will dig their heels in and say no.
So long as i can get mum to shower once a week and washes the rest of the time i am happy and its a bonus if i can get her to have more.
You could try to make it like a pampering session this sometimes can help.

Or she may prefer a bath! When my husband would not shower anymore, I offered to wash his back in the tub! He switched to tub baths and loved it!