There are a few reasons why your close contacts may not receive an app alert when you tested positive.
It could be one of the following:
The phones were not within 2 metres of each other for 15 minutes or longer. Read more on when the app sends an alert after being in 'close contact' with someone who has tested positive for coronavirus (COVID-19).
One, or multiple phones, had contact tracing paused. Read more about how and when to pause contact tracing.
The person that tested positive did not book their test through the app and did not enter the result on the app. Read about how the app knows your COVID-19 test result if you did not book the test through the app.
The person that tested positive chose not to share this using the app with recent 'close contacts'. Read more on how the app alerts people you've been in close contact with after a positive COVID-19 test result.