Your power is so low that the inserters are too slow to grab it until it gets out of reach.

Want to write you, that I saw this answer earler. But one second please, WTF o_O red color on electrical power diagram isn't good? Damn!
Man, thx. I look at boiler they work and I thing thats all ok. You ...

I try to find answer on forum, but doesn't find.
As I sayd in subject little problem with yellow Inserter - they can't grab items from the belt (tier 1, yellow). There is no curves, no turns but Inserters arm moves very slowly.
Bugs happened after 6 ingame hours, on start everything was good.