It could be that you need to use a low tension bobbin case, which is especially made for free motion quilting. How do I know, I own the Elna Excellence and need to use the low tension bobbin case. My Elna and your Horizon are very similar in the making.
To me it's an inconvenience and would have not bought the Elna, would I have known this before. (Which the dealer didn't mention before).
I hope you find the solution for your problem and will be able to enjoy your sewing machine soon.