Natasha did put up one hell of a fight. She battled as many of HYDRA's idiotic goons as possible. Eventually, the number of them overpowered her. She may be unstopped at times, doesn't mean she isn't human. It also doesn't mean she is giving into them completely. Zola had started to run experiments on her; she needed to find out what exactly he was up to. The only way to find out without letting him know is to let him do what he wanted.
Natasha did her best to endure all the pain she is feeling with the needles and everything else Zola is doing with her. He wanted to know what made her tick; he wanted to torture for the sheer pleasure of it all? Who knew really. She knew he wasn't up to no good.
Each day they would bring her down to see him and his team of crazed scientists doing odd, strange and painful experiments. Her body and mind are on the verge of giving in. With what strength she had, she was able to escape briefly to send out some kind of signal. She knew Clint and the others would pick up on a unique signal from her.
They finally got her; she had no strength to fight them off. "Rats, you found me boys," she said, before being picked up and dragged off to her cell.