I have a Horizon and love it, no problems so far. My old machine had the problem of making a rats nest underneath. They blamed it on improper threading, metal bobbin, or foot not down. I only had it for 20 yrs, so I thought I had this all down. I did find that sometimes it was not catching on the hook inside, where it runs thru the tension. Put the foot down, and pull the thread to see if it is pulling smoothly.
Or call your Janome man, someone there should be able to help you. My dealer told me I could come in anytime for a one on one. They told me to go home and get the basics and then come back for hands on with them.
I love the little man that hides behind the bobbin and cuts my thread.