are you using a 'lofty' batting or a thin batting? that can make a difference- if using a lofty batting it helps to lengthen your stitch length a little and if possible ease the foot pressure- you could try spray basting - some people find that makes a difference- I've always thread basted and used to have that problem at times- but once I started lengthening my stitch length and easing the foot pressure it helped a lot.