The long strand of yarn is really the crossed knit stitch, unless you're doing a YO between working the sts on the L needle and the cable needle, or somewhere else. Yes, it may be that you're knitting too tight. Don't work cables tight, keep them loose so they can stretch; if they're tight they'll pull on the adjacent sts and cause more gaps. Loosely knit the stitches will relax and fill in the gaps.
sue- knitting heretic