Much grateful thanks for your suggestions! Although I'd rethreaded the top thread multiple times, I hadn't thought to do the bobbin thread too, so have done that, and I've now changed the needle (which was new in, but I guess we can always get a dud, huh?). I don't have any 14 needles (I'm using size 12 quilting needles), but will stock up next time I can get out - and look into the topstitch needles as well.
I think drag around the needle may also be a factor (there's a lot more quilt to catch than in a little practise sandwich, isn't there!), and I am trying my best to manage that better.
And, fingers crossed, it seems to be working! Thank you again! As a novice quilter, I really, really appreciate the assistance people can offer.