are you using a needle for knits, and is it the right one for the weight of fabric you are sewing? (apologies if you already know this). there are different needles for lightweight wovens (silks, etc) heavyweights like denim, and needles for different weights of knit fabrics. i just noticed you mentioned ballpoint needles, so you probably already know this.
sometimes the feed dogs or the presser feet have a hard time handling knits, especially slinky slippery ones. you can also try sandwiching the fabric between two sheets of tissue paper...that has helped me before. the tissue paper can be easily torn away afterwards.
if you can spare some scraps of the knit, try different stitches & lengths, too.