Janome I have the 6600 and have had the same problem. !. the machine must be free of lint etc so clean reg.
2 There is a little tension spring that the cotton come in contact with as it goes to the take up lever mine had slipped out once I put it back it was ok. Had the machine to dealer twice and they couldnt find the problem . Rang a man I dealt with 30 years ago and he sorted it out. Also try another cotton Machines are funny dont like some cottons My friend Janome wont sew with their cotton Carmen