There should be a reminder about the jam ending on e-mail if I remember correctly. Sometimes people make games or assets in jams and life hits them or they just are not happy what they have made and thanks to that they have not submitted. I do know that developers are simple if there is an insentive to get something they are more likely to join and submit, maybe even if its just 5 dollars or recoqnition on social media or anything alike that, a good name for the jam.
You can also if I'm not wrong directly press on the user and message them through email (At least you could a year ago not sure now).
I have as well joined multiple jams made and focused on games, but just didnt like it so I ended up not submitting because I just felt bad submitting it, I think its fine. 3 Developers are already a huge sum I have seen a lot with nothing.