First, check whether the memory of the mobile phone or the memory of the mobile phone itself is full.

2.

See if the download directory of QQ music settings is set on the mobile phone memory card (I can't find it myself when downloading it to other parts of the computer).

3.

See if the downloaded songs can be downloaded (some songs have been deleted or dead)

4.

If there is nothing wrong with the above, check whether there is something wrong with your mobile phone (you can format your mobile phone's memory card before downloading it, or try installing it on another mobile phone. If you don't succeed, it is recommended to go to the mobile phone repair office to see if there is something wrong with your mobile phone or memory card.