Failed Attempt at Discipline

I'm a 35-year-old trans woman and I've been in a DDLG relationship for the past year. I play Mommy to my little girl, who is also in her late twenties. We had our ups and downs but we found a good balance between playful discipline and nurturing care. Recently, there was an incident where she broke one of the rules we established about personal hygiene. After that, I decided it was time for some tough love. I planned a little lesson for her involving spankings and grounding until she apologized properly. I set up a spanking bench in our basement - complete with soft padding, fluffy blankets to lie on afterwards, and an assortment of implements. It looked pretty scary, actually. She didn't flinch when she saw it all prepared; instead, she smirked at me. The actual discipline went...poorly. She refused to cry or beg for mercy no matter how hard I hit her - it was like hitting a brick wall. Instead of breaking down and learning from the spanking, she remained defiant until I gave up. It just didn't feel right. So here's my question: Was this normal? Should I try again with a different approach or should I back off entirely on that part of our dynamic? She says she loves me and wants to continue but I'm feeling pretty lost about how to handle it properly.