Sometimes the paper can be a bit slippery and cause the puckering. Do you happen to have a teflon foot? I would also try adjusting the foot pressure to make sure the feed dogs are moving the paper along at the same speed as the fabric. I don't have a Janome 15000, but I do have the 8900. Are you using the Accufeed feet or the standard feet? That might also be a factor. If you're using one, try the other to see if that helps.