Stagansi Zanketsu
It happens sometimes when people go around and catch them all. Go to an area that doesn't have any vacancies...like where people usually wouldn't go. There'll probably be some there.
Edit: Yeah, I tried this method, and I caught like 6 bugs in two minutes. Just go where other people don't.
But I'm always in places where no one is. That works for garbage and flowers and stuff, but I'm saying I haven't seen a single bug in four days, populated area or non. I've even had people walking next to me who have had their bug catching moments, dry spells, and bug cathing moments again while they've walked with me. It's not merely that I can't find them. They're not there at all.