Overflow Stopped Working. Ideas why?

So, I currently use two overflows on my tanks. One in my 650gal and one in my 350gal. Both stop working sometimes and I haven't a clue why. It's a pain in the ass. Its a random event; weeks in between, sometime. If they stop working, the ladies get deficiencies, and fish die. Any ideas very welcome.

Below is how it should work.

Do they share a common vent? Possible airlock if they do?

No, the other tank's overflow is across the room. They do drain to a common return, but that's after the breather vent.