Problem with occasional loops on the topside of my leather with the singer 29 k 33

So I've bought and restored an old singer machine. A singer 29k33 to be exact. But it will occasionally not pull the top thread tight. Ive changed the tension (top and bottom) but to no avail. The two threads do meet in the middle but the top just sometimes doesn't pull itself tight. I've introduced new tension points and let go of some tension points still no change. Can anyone help with this?