I have noticed a lot of problems relating to disc pivot lately and have encountered it in a few lessons lately.
I think most people don't understand the disc pivot. I believe most people think you should allow the disc to pivot around the index finger in the hand by gripping loosely and letting it swing around.
that is false and will most certainly lead to micro slipping, if not major slipping.
the pivot does happen a little bit. it is not something you try for, it is incidental. It happens because you do other things correctly.
allowing for a loose pivot is pretty counter-productive. the pivot yields strength to the throw because it doesn't happen loosely.
the outer edge of the disc SHOULD come around. it SHOULD come around because you are pulling it around. read that again: pulling it around. that is not the same as letting it swing around.