Inserter wouldn't load second type of item

[TheAlterMind]

Hi! I'm not sure what is the problem, but what I see on the picture below is:
- blue inserter should load onto the train either coal or empty barrels
- the wagon has filters so there are designated spots for both coal and empty barrels
- there are empty barrels coming along the belt, but the inserter does not load them
- what I see is that the inserter is holding a piece of coal, but there is no room left on the wagon for coal
- the other two inserters work fine although they do the same

Can you point me to what the problem is and maybe suggest solutions? Thanks in advance

[Serenity]

You already said exactly what the problem is here. Inserters can grab more items than what they can fit in the cargo wagon. So they are stuck with that item in their hand.
Either limit the inserter stack size to one. Or better, use one filter inserter per item type

[TheAlterMind]

Thank you so much. I'm still learning the game. I was wondering what is the purpose of this stack limiting. Now it works fine.

[Serenity]

Keep in mind that that obviously lowers the throughput. That's fine in some situations, but it can also be too slow. If you need the speed then several inserters are better