For some reason your doe may not be fertile. It happens. When I was starting out and did not have my own buck, I would travel several hours to a dairy, and pick out the buck I wanted. He would breed her once or maybe twice and five months later I had babies.
Just a comment. If the doe is in heat and then returns to heat five days later, the first heat is not fertile but the second usually is. I would see those five day heats occasionally when the does first started to cycle at the start of the breeding season. When I had my dairy, does like yours didn't stay around.
I agree with this post 100%.
If you have tried to breed her that many times she probably won’t get pregnant. I worked at a goat dairy for a few years and this happened to a young doe (coincidentally a Nigerian Dwarf too) there. It was sad as she was a nice doe and destined to be a great little milker. But she had to be sold to a pet home. The owner tried for 2 breeding seasons to breed her, with multiple bucks, and even penned her separate for 2 months with a buck and nothing.
Maybe try and find a pregnant doe for sale?