@Patman To clarify, are you referring to a playlist you already have downloaded for Offline Mode?
For example, if you add songs to a playlist online (that is already downloaded for Offline Mode), those new tracks should immediately start downloading for your offline playlist as well.
Depending on how many tracks you are adding, you may need to wait a little to give those tracks time to download.
You can then confirm they were added to your offline playlist by switching over to Offline Mode.
Let me know if this helps.
