Some battery saver or battery optimisation tools on your phone might stop exposure notifications working.
To check whether this is the case, go to the COVID-19 Exposure Notifications setting on your phone and look at Exposure Checks. If you see gaps in the log during times that you were using a battery optimisation tool, it is likely that the battery optimisation tool is affecting the NHS COVID-19 app.
To fix this, go into the settings of your battery optimisation tool (this could be in your phone’s settings or in the settings of a separate app). In most cases, you can exclude specific apps from battery optimisation. If you exclude the NHS COVID-19 app from being turned off during optimisation, exposure notifications should continue to work, even when you're using battery optimisation. You should be able to do this by selecting Don’t optimise or similar.