Try following steps respectively and see if the problem is solved.

1) Update KKBOX Android.
Look up [KKBOX] in Google Play and ensure you have the latest KKBOX Android installed.
Once you have updated to the latest version, try downloading the tracks again.

2) Change the location of your Offline Songs.
Tap [More] → [Settings] → [Offline Songs Storage Location] and change the location of your Offline Songs (from internal storage to SD card, vice versa), and try downloading the tracks again.


3) Change the Offline Song Capacity.
Tap [More] → [Settings] → [Offline Song Capacity] → choose [4,000 songs], and try downloading the tracks again.


4) Re-download the tracks.
Delete the tracks you are having trouble with and download them again.

5) Restart your device.
Restart your device and try downloading the tracks again.