My normal photo taking procedure is to take an excess of photos on my Android phone, allow them to be automatically uploaded to Google Photos, and to then delete the unwanted photos through the Google Photos website on my PC, which would then delete them from my phone too. After a recent update to my phone I found that photos were no longer being removed from it.
The trick was that the Google Photos app on my phone had not been granted permission to perform the deletions. I had to manually delete a photo on the Google Photos app to prompt it to ask me for the necessary permissions. After acquiescing, the deletions then flowed through from the web, though only for photos deleted since making this change.