I had the same problem yall, and here is the ultimate solution!!! I found it!
In your playlist, you must have clicked the top bar that says: "Name" or something, At least that's what I clicked and immediately after, I wasn't allowed to re-arrange my songs.
The solution:
To your left of that bar, there is another column, with numbers, giving you the numbers of how many songs are in your playlist. Click that, and you should see an arrow facing UP or DOWN.
So
1. Clicking the "NAME" bar, will lock the playlist order.
2. Clicking the "NUMBER" bar (not TRACK NUMBER, the bar "left-est")
Arrow Up=Unlock, then custom arrangement.
Arrow Down=Flips that list.
Now if you click the "NAME" bar, it will look, in alphabetically order.
Click back the arrow to re-arrange again.
Play with both of them a few times, back and forth, that way, you'll remember!
Spread the word!

Peace. ///
Let me know if it worked for anybody. ///