I use Hobbs 80/20 most of the time. The only problems I have noticed is when I buy the packaged, it is compressed more in some areas and even stretched a bit. I find this same problem with a lot of packaged batting bought by the piece. Now, I haven't bought any for a few years as I had a roll of the 120 and a roll of the 96 wide as well as a roll of polydown which I like a lot too. I haven't noticed any thin spots in any of these rolls.
That was the 80 cotton and 2o poly blend. What "mix" did you use?