Definitely try some heat-n-bon! Knit is notorious for being stretched a bit by the feed dogs of your machine as it's sewn. Makes it seem like your ribbon 'shrunk' but it was actually the knit stretching. If you know how (and it's not a lot of trouble) you may want to loosen your presser foot tension just a bit so the feed dogs don't stretch the material as much.
This is why sergers are usually used to sew knits - regular machine pressure tension can be too much and stretch stuff all out of whack.