I really don't know, but are you sure your bobbin is wound well? A lot of issues crop up when a bobbin is not wound tightly and evenly. If you have a pre-wound bobbin available, you might try that to see if it solves the problem.
I have seen quite a few comments online about the Serenade having a variety of issues relating to the bobbin that seem associated with the bobbin sensor system. You can do a search on the QB and Google to find them. Someone posted that it got bad reviews on the patternreview.com website, but I haven't read them.