The 1600 isn't a top loading bobbin.

I have a 1600P QC and the HL needle is the exact one recommended for the machine. I doubt it is the needle alone, unless it is the fact that your fabric is someone really dense and the needle is trying to slide between the threads, and needs to just "pierce" the threads.

--my machine has been "trouble-free" so far (so grateful)...in fact, it's lovely AND fast.

But, try all that people have suggested including making the bobbin and top thread match (sometimes I don't have them matching) and a Superior titanium topstitch needle with a larger eye (I've done that when I broke me last HL needle and didn't have anymore..it worked just fine.)

Is there anything unique about the fabric you are using? Is it an especially tight weave?

I just went through this with my Viking, and found out that the needle was inserted just a tiny bit short of being set properly, between that and the needle size I went to an 80, It finally fixed the problem

That was my first thought. I've got a Janome, too, and whenever I've got stitches skipping it's always because the needle is just a tad bit off.

I got a blue bobbin for fmq and leave it in all the time. Is that a mistake? I have no issues using it but makes me wonder.

Hoping to get the top stitch needles in the mail very soon.

I had same problem when I first had my 8900. Drove me mad - it skipped about every 10 or 12 stitches. Then I read about the Janome purple needles from others who have had the problem. Brilliant, goes like a dream ... takes any thread and give beautiful stitches. I found the Organ equivalent and use those as well (the purple tipped Janomes are made by Organ anyway). Organ needles (& the Janome purples) are very marginally longer than the Schmetz needles. My 8900 will not stitch at all well with Schmetz, only Janome/Organ branded, and always topstitch for free motion.