For some reason some of my inserters some times skip picking up resources.  
I've tried to record this, but then all of a sudden they pick up resources every time they can...  
I've only noticed this with assembling machines, but the input stacks aren't full and nor are the output stacks. Yet the inserter will notice ingredients coming down the belt and skip them.  

I've made a recording of this happening with Science pack 2 not being inserted in labs while researching Toolbelt, which requires Science pack 2. We can see that the labs aren't active, so it's not due to it being full.  

My installation is up to the latest Steam version without any mods.  
  
Any ideas what might be causing this?

Potential problems that aren't occuring:  
- I'm using yellow belts, this is the slowest belt in the game.  
- It's happening with straight belts, no corners involved.  

What might be the case:  
- The belts are nearly empty, it feels very inefficient to have full belts by design. Like the different parts of the factory aren't properly tuned.