Psychological

Ultrama'am is powerless in the presence of ultranite not because of anything about ultranite - it's because she has mental trauma from the destruction of her island nation. When she has a strong reminder that everyone she loved died violently. It shakes her to her core, and her powers fizzle out as a result. Ultimately, it's an irrational phobia. If she doesn't know it's there, or if she believes it is 'properly contained' behind tungsten (which also involves it being out of sight... because a psychologist figured it out and convinced Ultrama'am that it worked that way).

Many supers suffer from such things, often related to the circumstances where their powers first manifested.