If an inserter picks up an item that is not meant to go into the specific production building he is connected to, he will be blocked for a couple of seconds. That might be what youre witnessing. The solution is to plan conveyor belts accordingly. Cheers
Yeah I realized that I wasn't actually doing it properly. Although I do think it'd be nice for a little bit of a hint or two for how some things work just for people to understand what's happening.