auto-sweepers refuse to pick-up materials from receptacles

I encounter a weird situation, and can't understand why it was not working properly. Here's the conveyors system :

As you can see, the receptacle is full of materials.

The auto-sweeper above is set to pick materials and try to put it into the loader on is left but is also commanded by automation so that it drop the material on the ground.

Sometimes, it manages to put it into the loader, so I've set this rail to send it back into the receptacle.

BUT ! to make the system work, I have to modify it this way :

As soone as the upper rail is cut, the system restart properly. And with the bridge, it also work properly.

So ... wath's the issue with the first system ?

Even the sweeper below the receptacle don't work to dispatch materials among the loaders.

It's not a matter of priority

Astraea.sav