one thing, did you bring the bobbin thread up to the top before doing your quilting? You need to do that, hold the threads and take a few tiny stitches (barely moving the quilt) and then trim the threads when you can.
that should help. a clean machine helps a lot too.
zero stitch length is good but not necessary. the movement of the quilt determines the length but the mess on the underside is another thing.
try cleaning and stitch something regular first to see if all is back to normal.
then try free motion on some scraps with batting in between.
good luck