Although, I've never really bred bettas, I have 2 in my community tank. To make a long story short, my dad bought 2 of them, 1 for each tank. But it turns out my polys liked their tails too much so he placed them both in the community tank temporarily. But they fight too often when they're together, so he places one in a net for 1-2 days at a time and switches them afterwards.
But what I noticed is that whenever, they're in the net they'll make bubbles nests and whenever they're in the huge community they just swim around. I'm guessing that they feel alot more secure in the smaller space. So try placing them back into the jar or at least a tank with more cover. Maybe that'll work. (My opinion only of course)