Not all L bobbins and size 15 are the same.  I’ve always found it best to use only Janome bobbins, supplied in the Janome blister pack.  I did try size L prewound bobbins and the are slightly smaller than a Janome bobbin, they bounce and clatter in the machine.  I have used Janome prewound bobbins, they had a better quality thread and less prone to causing pick up issues when half full, the thread in the non Janome bobbins tended to pull back and curl because of the nature of the thread.  Janome thread doesn’t do that it’s good to the point at which the machine warns it’s getting low.