The instructions say:
After 15 minutes of development time, illuminate the result window
with a UV flashlight to observe the test result.
I assume that with the first test you allowed the development time and took it to a dark place where you could see the UV light. If not, it may just be that there was too much light to see the UV illuminated result.
The TGA rate the Hough RAT as high sensitivity. This means:
For each test a comment on the clinical sensitivity is assigned as follows:
Acceptable sensitivity - clinical sensitivity greater than 80% PPA
High sensitivity - clinical sensitivity greater than 90% PPA
Very high sensitivity - clinical sensitivity greater than 95% PPA
PPA is the proportion of individuals who produced a positive test result using a COVID-19 rapid antigen test in comparison with a positive result obtained using a more sensitive laboratory PCR test.
To me this means that 10% of the Hough RATs will not pick up a positive case. If you looked at the first test in a dark space, you may well have got one of the 10% that doesn’t pick it up.