First of all, Nesie, have no fear, your 6600 it can handle the thicker batts!
It's an amazing machine.
Some thoughts ....
Because you are talking stitch quality issues, one of my first thoughts is the needle.
Try different types of needles as well as sizes.
One of my favourites for FMQing (and even regular stitch quilting) is an embroidery needle.
Sizes 12 and 14 are usually my go to's for starters.
Though others will say their go to's are different for both type and size!
If FMQing, you may need to adjust the foot to have a bit more ease of movement between the FMQ foot and the machine bed.
While you don't think the crib quilt would cause much drag, it can cause some.
So definitely something to consider, as to how you have it in position/draped while you quilt.
Both of the above are the first things that I would consider based on your description of the stitches.
Tension is something else that could need to be changed when switching from one batt to another.
Peckish will probably have some other ideas too!
Nesie, Let us know! Please?
Last edited by QuiltE; 08-06-2024 at 05:09 PM.